Highly active photocatalyst particles, method of production therefor, and use thereof

ABSTRACT

A method of producing composite particles of titanium dioxide and a compound inactive as a photocatalyst, comprising the steps of preparing a water based slurry of pH 3 to 5 comprising titanium dioxide, preparing a water based solution comprising a compound inactive as a photocatalyst, and reacting the slurry and the water based solution together at a pH within a range from 4 to 10 is provided, together with highly active photocatalyst particles produced using such a method, and potential uses of such photocatalyst particles.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of Provisional Application No.60/392,970 filed Jul. 2, 2002; the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photocatalyst with highphotoactivity. More specifically, the present invention relates tophotocatalyst particles and powder which are capable of exhibiting goodphotocatalytic function using a practical light source of extremely lowintensity, such as a fluorescent lamp, and also relates to an organicpolymer composition, a slurry, a coating agent and a film which displaysboth photocatalytic properties and hydrophilicity, incorporating such aphotocatalyst, and articles using the same.

2. Description of Related Art

Conventionally, titanium oxide has been widely used as a typical,practical photocatalyst. Titanium oxide has the property of absorbingultraviolet light at wavelengths below approximately 400 nm andproducing an excited electron. When the generated electron and holereach the particle surface, combinations with oxygen and water and thelike generate a variety of different radicals. These radicals typicallycause an oxidizing action, and oxidize and decompose substances adsorbedto the surface thereof. This is the basic principle of photocatalysis.The use of the optical functions of ultra fine particles of titaniumoxide in antibacterial, deodorizing and stainproofing applications, andin environmental clarification [clean-up] applications such asatmospheric purification and water quality purification are currentlyunder investigation.

Examples of methods for maximizing catalytic function include themethods described below.

A method for,

-   (1) reducing the particle size (This method is extremely effective    in suppressing the recombination of the generated electron and    hole.);-   (2) increasing the crystallinity (This method is effective in    raising the speed with which the generated electron and hole diffuse    towards the surface.);-   (3) performing charge separation (This method involves charge    separation of the generated electron and hole, to increase the yield    of electrons and holes which reach the surface.); and-   (4) adjusting of the band gap.

If the band gap is reduced (and the maximum wavelength of absorption isincreased) by the addition of a minute quantity of an impurity, then theefficiency of light sources emitting little ultraviolet light, such asthe sun and fluorescent lamps, can be improved.

Of these methods, in recent years, the investigation of so-calledvisible light responsive photocatalysts, aimed the method (4) above, hasattracted considerable interest.

For example in Japanese Unexamined Patent Application, Laid-open No. Hei9-262482, the maximum wavelength of light absorption for titaniumdioxide was shifted to a longer wavelength by modification of an anatasetitanium dioxide with high catalytic activity through ion injection ofmetal elements such as Cr (chromium) or V (vanadium), thereby producinga titanium dioxide capable of catalytic action under visible lightirradiation. However, this type of ion injection of metal atoms requiresa large apparatus and is expensive, meaning that the industrialpracticality is limited.

In addition, Japanese Unexamined Patent Application, Laid-open No.2001-72419 discloses a titanium oxide with an index X=B/A of no morethan 0.97, wherein A represents the average of the half-width of thetitanium peak at the first and second measurements among fourmeasurements of the half-width of a titanium peak of titanium oxide witha bond energy within a range from 458 eV to 460 eV as measured by X-rayphotoelectron spectroscopy, and B represents the average of thehalf-width of the titanium peak at the third and fourth measurements.However, not only is the activity of the powder unsatisfactory, but thepowder is also colored, meaning that the potential applications of thepowder are limited. In a practical sense, the powder also has otherdrawbacks, such as being unsuitable as a coating in which transparencyis required.

Furthermore, many of the conventional visible light responsivephotocatalysts require the use of a powerful light source such as axenon lamp in order to ensure an adequate catalytic function, which,needless to say, reduces their practicality. A photocatalyst capable ofexhibiting an adequate effect with a low cost light source, for example,a typical indoor light source such as a day white fluorescent lamp,would have considerable merit.

International Patent Application, No. WO94/11092 discloses a method oftreating bacteria and malodorous substances by applying a photocatalyticthin film formed from a semiconductor such as titanium dioxide on theinternal walls of a hospital ward or living spaces, although no mentionis made of the method of producing activity within the titanium dioxide,nor of the photocatalytic activity of the particles. If normal titaniumdioxide is used, then it is envisaged that the activity achieved using alight source with a very low proportion of ultraviolet light such as afluorescent lamp would be even lower than that achievable using thevisible light responsive photocatalyst described above.

Furthermore, a representative example of applications focusing on thephotocatalytic function of fine particles of titanium oxide includemethods of kneading fine particles of titanium oxide into a substratesuch as an easy handling fiber or plastic molded product, or methods ofapplying fine particles of titanium oxide to a substrate such as clothor paper. However, the powerful photocatalytic action of titanium oxidecauses the decomposition not only of harmful organic materials andenvironmental pollutants, but also of the fiber, plastic or paper mediumitself, meaning these types of medium are prone to deterioration and areunable to present a practical degree of durability. In addition, becauseof the ease of handling offered by fine particles of titanium oxide,paints comprising a mixture of fine particles of titanium oxide with abinder are being developed, but a durable and low cost binder capable ofovercoming the above deleterious effects on the medium has yet to befound.

Japanese Unexamined Patent Application, Laid-open No. Hei 9-225319 andJapanese Unexamined Patent Application, Laid-open No. Hei 9-239277disclose measures for suppressing or preventing the deterioration of aresin medium or a binder resulting from the powerful photocatalyticaction of titanium oxide, and propose methods in which a photo-inactivecompound, which comprises aluminum, silicon or zirconium, is supportedon the surface of particles of titanium oxide in the form of islandswith steric hindrance, thereby suppressing the photocatalytic action ofthe titanium oxide. However, although this method results in thesupporting of islands of a photo-inactive compound on the photocatalyst,specific regions of the resin medium or binder are still exposed to thepowerful photocatalytic action of the titanium oxide.

Japanese Unexamined Patent Application No. Hei 10-244166 (Laid-open No.Hei 11-335121) proposes a photocatalytic titanium oxide in which thesurface of the titanium oxide has been coated with a film of porouscalcium phosphate, although in this case, the coating of calciumphosphate causes a reduction in the photocatalytic performance of thecatalyst.

International Patent Application, No. WO99/33566 discloses a fineparticulate powder of titanium dioxide in which a porous calciumphosphate layer is formed on at least a portion of the surface of thetitanium oxide particles, with an anionic surfactant provided at theinterface therebetween.

Furthermore, Japanese Unexamined Patent Application, Laid-open No.2002-1125 discloses a photocatalyst powder comprising fine particles oftitanium dioxide containing an anionic active substance such ascondensed phosphoric acid, wherein the interfacial potential of the fineparticles in a water based environment of pH 5 is within a range from 0to −100 mV.

In addition, in terms of slurries comprising titanium oxide withphotocatalytic activity, Japanese Unexamined Patent Application,Laid-open No. Hei 10-142008 discloses an anatase titanium oxidecontaining slurry produced by subjecting a titania sol solution, atitania gel or a titania sol/gel mixture to beat treatment andsimultaneous pressure treatment inside a sealed vessel, and subsequentlydispersing the product with ultrasonic waves or by mixing.

Furthermore, Japanese Unexamined Patent Application, Laid-open No. Hei11-343426 discloses a photocatalyst coating with excellent dispersionstability, wherein the photocatalyst coating comprises titanium oxidewith a Raman spectrum peak within a range from 146 to 150 cm⁻¹ and inwhich the proportion of anatase titanium oxide is at least 95 mass %,and silica sol, in a solvent.

However, the isoelectric point of the titanium oxide is from 5 to 6, andat pH values close to neutral, namely from pH 5 to 9, the titanium oxideis prone to aggregation, and obtaining a stable, highly transparentdispersion (slurry, sol or the like) in a solvent is difficult.Accordingly, dispersions in the acidic region are typically used,although such dispersions have undesirable effects on living organismsand the environment, and exhibit a corrosive action on metals, whichcannot be ignored, making the dispersions unsuitable for use on metalsubstrates. Consequently, a neutral, stable titanium oxide sol has beenkeenly sought.

Japanese Unexamined Patent Application, Laid-open No. Hei 11-278843discloses a titanium oxide sol of pH 5 to 10, comprising 50 to 100 partsby weight of negatively charged titanium oxide colloidal particlescomponent, 5 to 50 parts by weight of a chelating [complexing] agent,and 1 to 50 parts by weight of an alkaline component. Furthermore,Japanese Unexamined Patent Application, Laid-open No. 2000-290015discloses a method of producing a neutral titania sol with transparencyand dispersion stability in the neutral region and formed fromdeflocculated titanium oxide particles covered with a hydrated phosphatecompound, by mixing a titania sol obtained by deflocculating hydroustitanium oxide with a water soluble titanium compound and a phosphatecompound, and removing acid from the reaction liquid. In addition,Japanese Unexamined Patent Application, Laid-open No. Hei 7-89722discloses a method in which a neutral titanium dioxide sol is stabilizedwith a hydroxycarboxylic acid or a derivative thereof, wherein prior to,during, or after the stabilization, the titanium dioxide sol is treatedwith a metal ion, an inorganic anion, a chelating [complexing] agentand/or an oxidizing agent.

As described above, a number of techniques have been disclosed, althoughthe conventional technology to date has not been able to providephotocatalytic particles capable of exhibiting good photocatalyticfunction using a practical light source of extremely weak intensity suchas a fluorescent lamp, while retaining good durability and dispersionstability in those cases when the photocatalyst is used with an organicmaterial, nor an industrially useful method of producing a neutral,highly transparent slurry containing these types of particles.

SUMMARY OF THE INVENTION

The present invention takes the above factors into consideration, withan object of providing a method of producing photocatalyst particlescapable of exhibiting good photocatalytic function using a practicallight source of extremely low intensity such as a fluorescent lamp, aswell as photocatalyst particles and powder, an organic polymercomposition using such particles or powder, a neutral and highlytransparent slurry and coating agent containing such particles, a filmdisplaying photocatalytic properties formed therefrom, and articlescontaining such photocatalyst particles. Furthermore, the presentinvention also provides the above types of compositions and films inwhich there is little coloring, and films which are highly transparent.

In addition, one aspect of the object of the present invention providesa photocatalytic powder and slurry which offers a considerableimprovement in industrial usability by displaying excellent dispersionstability, while suffering no loss in the photocatalytic activity oftitanium dioxide, as well as a polymer composition, a coating agent, aphotocatalytic molded article and a photocatalytic structural bodyformed using the powder or slurry.

In addition, the present invention provides a photocatalytic powder andslurry which, when applied to the surface of a fiber, paper or plasticmaterial, or kneaded into such a material, or used in a coatingmaterial, exhibits excellent photocatalytic activity, durability anddispersion stability.

As a result of intensive research aimed at achieving the above object,the inventors of the present invention discovered that by complexingfine particles of titanium dioxide and a photocatalytically inactivecompound such as a condensed phosphate salt under specific conditions,particles of the present invention could be produced. The inventorssubsequently produced a slurry using these particles, and by using thisslurry, were able to achieve the aforementioned objects.

In other words, the present invention relates to the aspects [1] to [88]described below.

-   [1] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst, comprising the steps of    -   preparing a water based slurry of pH 3 to 5 comprising titanium        dioxide,    -   preparing a water based solution comprising a compound inactive        as a photocatalyst, and    -   reacting the slurry and the solution together at a pH of 4 to        10.-   [2] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to aspect 1    above, wherein the concentration of titanium dioxide in the water    based slurry comprising titanium dioxide is 5 mass % or less. p1 [3]    A method of producing composite particles of titanium dioxide and a    compound inactive as a photocatalyst according to either one of    aspect 1 and aspect 2 above, wherein the concentration of titanium    dioxide on mixing of the water based slurry comprising titanium    dioxide and the water based solution comprising a compound inactive    as a photocatalyst is no more than 5 mass %.-   [4] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 3 above, wherein the reaction temperature    between the water based slurry comprising titanium dioxide and the    water based solution comprising a compound inactive as a    photocatalyst is no more than 50° C.-   [5] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 4 above, wherein the step of preparing the    water based slurry comprising titanium dioxide includes a process    for the wet synthesis of titanium dioxide, and does not include a    process for producing titanium dioxide powder from the synthesized    slurry.-   [6] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 5 above, wherein the titanium dioxide    comprises an anatase crystal form.-   [7] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 6 above, wherein the titanium dioxide    comprises a brookite crystal form.-   [8] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 7 above, wherein the titanium dioxide    comprises a rutile crystal form.-   [9] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 8 above, wherein the titanium dioxide    comprises at least two crystal forms of anatase, rutile and brookite    forms.-   [10] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 9 above, wherein the BET specific surface    area of the titanium dioxide is within a range from 10 to 300 m²/g.-   [11] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 10 above, wherein the compound inactive as a    photocatalyst is a salt selected from a group consisting of    phosphates, condensed phosphates, borates, sulfates, condensed    sulfates and carboxylates.-   [12] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 11 above, wherein the condensed phosphate is a salt selected    from a group consisting of pyrophosphates, tripolyphosphates,    tetrapolyphosphates, metaphosphates and ultraphosphates.-   [13] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 10 above, wherein the compound inactive as a    photocatalyst is at least one compound selected from a group    consisting of Si compounds, Al compounds, P compounds, S compounds    and N compounds.-   [14] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to any one of    aspect 1 through aspect 13 above, wherein the compound inactive as a    photocatalyst comprises at least one metal selected from a group    consisting of alkali metals, alkali earth metals, transition metals    and Al.-   [15] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to aspect 14    above, wherein the alkali metal is at least one metal selected from    a group consisting of Na and K.-   [16] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to aspect 14    above, wherein the alkali earth metal is at least one metal selected    from a group consisting of Mg and Ca.-   [17] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst according to aspect 14    above, wherein the transition metal is at least one metal selected    from a group consisting of Fe and Zn.-   [18] A method of producing composite particles of titanium dioxide    and a compound inactive as a photocatalyst in which the titanium    dioxide is surface treated with the compound inactive as a    photocatalyst, wherein the composite particles display a higher    photocatalytic activity than the raw material titanium dioxide.-   [19] Composite particles of titanium dioxide and a compound inactive    as a photocatalyst, produced by a method according to any one of    aspect 1 through aspect 18 above.-   [20] Composite particles of titanium dioxide and a compound inactive    as a photocatalyst according to aspect 19 above, wherein the    compound inactive as a photocatalyst exists partially on the surface    of the titanium dioxide.-   [21] A water based slurry comprising composite particles of titanium    dioxide and a compound inactive as a photocatalyst, produced using a    method according to any one of aspect 1 through aspect 18 above.-   [22] Photocatalyst particles comprising particles according to    either one of aspect 19 and aspect 20 above, wherein when 3.5 g of    the photocatalyst particles spread uniformly across a flat surface    of diameter 9 cm, placed within 5 L of dry air containing 20 ppm by    volume of acetaldehyde, is irradiated with a day white fluorescent    lamp producing an ultraviolet light intensity of 6 μW/cm² at a    wavelength of 365 nm, the ratio of decomposition of the acetaldehyde    after one hour of irradiation is at least 20%.-   [23] Photocatalyst particles according to aspect 22 above, wherein    the ratio of decomposition is at least 40%.-   [24] Photocatalyst particles according to aspect 22 above, wherein    the ratio of decomposition is at least 80%.-   [25] Photocatalyst particles according to aspect 24 above, wherein    the BET specific surface area of the composite particles of titanium    dioxide and the compound inactive as a photocatalyst is within a    range from 10 to 300 m²/g.-   [26] Photocatalyst particles according to aspect 25 above, wherein    the titanium dioxide comprises an anatase crystal form.-   [27] Photocatalyst particles according to aspect 25 above, wherein    the titanium dioxide comprises a brookite crystal form.-   [28] Photocatalyst particles according to aspect 25 above, wherein    the titanium dioxide comprises a rutile crystal form.-   [29] Photocatalyst particles according to aspect 25 above, wherein    the titanium dioxide comprises at least two crystal forms of    anatase, rutile and brookite crystal forms.-   [30] Photocatalyst particles according to any one of aspect 25    through aspect 29 above, wherein the compound inactive as a    photocatalyst is present in a quantity within a range from 0.01 to    50 mass % based on the mass of the titanium dioxide.-   [31] Photocatalyst particles according to aspect 30 above, wherein    the compound inactive as a photocatalyst is a salt selected from a    group consisting of phosphates, condensed phosphates, borates,    sulfates, condensed sulfates and carboxylates.-   [32] Photocatalyst particles according to aspect 31 above, wherein    the condensed phosphate is a salt selected from a group consisting    of pyrophosphates, tripolyphosphates, tetrapolyphosphates,    metaphosphates and ultraphosphates.-   [33] Photocatalyst particles according to aspect 30 above, wherein    the compound inactive as a photocatalyst is at least one compound    selected from a group consisting of Si compounds, Al compounds, P    compounds, S compounds and N compounds.-   [34] Photocatalyst particles according to aspect 30 above, wherein    the compound inactive as a photocatalyst comprises at least one    metal selected from a group consisting of alkali metals, alkali    earth metals, transition metals and Al.-   [35] Photocatalyst particles according to aspect 34 above, wherein    the alkali metal is at least one metal selected from a group    consisting of Na and K.-   [36] Photocatalyst particles according to aspect 34 above, wherein    the alkali earth metal is at least one metal selected from a group    consisting of Mg and Ca.-   [37] Photocatalyst particles according to aspect 34 above, wherein    the transition metal is at least one metal selected from a group    consisting of Fe and Zn.-   [38] Photocatalyst particles according to any one of aspect 30    through aspect 37 above, wherein the isoelectric point determined    from the zeta potential measured using an electrophoresis light    scattering method is no more than 4.-   [39] A photocatalytic powder containing photocatalyst particles    according to any one of aspect 30 through aspect 38 above.-   [40] An organic polymer composition containing photocatalyst    particles according to any one of aspect 30 through aspect 38 above.-   [41] An organic polymer composition according to aspect 40 above,    wherein the organic polymer of the organic polymer composition is at    least one polymer selected from a group consisting of thermoplastic    resins, thermosetting resins, synthetic resins, natural resins and    hydrophilic polymers.-   [42] An organic polymer composition according to aspect 40 above,    wherein the organic polymer composition is at least one type of    organic polymer composition selected from a group consisting of    paints, coating compositions, compounds and master batches.-   [43] An organic polymer composition according to any one of aspect    40 through aspect 42 above, comprising from 0.01 to 80 mass % of the    photocatalytic powder based on the total mass of the composition.-   [44] A photocatalytic molded article formed from an organic polymer    composition according to any one of aspect 40 through aspect 43    above.-   [45] A photocatalytic molded article according to aspect 44 above,    wherein the photocatalytic molded article is a molded article of at    least one material selected from a group consisting of fibers, films    and plastics.-   [46] An article produced from a photocatalytic molded article    according to aspect 45 above.-   [47] An article with photocatalyst particles according to any one of    aspect 30 through aspect 38 above on the surface thereof.-   [48] An article according to either one of aspect 46 and aspect 47    above, wherein the article is at least one article selected from a    group consisting of building materials, machinery, vehicles, glass    products, electric appliances, agricultural materials, electronic    equipment, tools, eating utensils, bath goods, toilet goods,    furniture, clothing, fabric products, fibers, leather goods, paper    products, sports goods, bedding, containers, spectacles, signboards,    piping, wiring, brackets, hygiene materials and automobile goods.-   [49] A slurry comprising photocatalyst particles according to any    one of aspect 30 through aspect 38 above.-   [50] A slurry comprising photocatalyst particles, wherein a powder    produced by drying the slurry is photocatalyst particles according    to any one of aspect 30 through aspect 38 above.-   [51] A slurry according to either one of aspect 49 and aspect 50    above, comprising water as a solvent.-   [52] A slurry according to either one of aspect 49 and aspect 50    above, comprising from 0.01 to 50 mass % of photocatalyst particles.-   [53] A slurry according to either one of aspect 49 and aspect 50    above, wherein the pH of the slurry is within a range from 5 to 9.-   [54] A slurry according to aspect 53 above, wherein the pH of the    slurry is within a range from 6 to 8.-   [55] A slurry according to any one of aspect 49 through aspect 54    above, wherein the light transmittance of the slurry, when measured    on a slurry with a photocatalyst particle concentration of 10 mass    %, using a wavelength of 550 nm and an optical path length of 2 mm,    is at least 20%.-   [56] A slurry according to aspect 55 above, wherein the light    transmittance is at least 30%.-   [57] A coating agent which produces a film with photocatalytic    properties, comprising photocatalyst particles according to any one    of aspect 30 through aspect 38 above, and at least a binder.-   [58] A coating agent which produces a film with photocatalytic    properties, comprising a slurry according to any one of aspect 49    through aspect 56 above, and at least a binder.-   [59] A coating agent according to either one of aspect 57 and aspect    58 above, wherein the binder comprises an organic compound.-   [60] A coating according to aspect 59 above, wherein the organic    compound is at least one organic compound selected from a group    consisting of acrylic silicon, polyvinyl alcohol, melamine resin,    urethane resin, acrylurethane, celluloid, chitin, starch sheet,    polyacrylamide and acrylamide.-   [61] A coating agent according to either one of aspect 57 and aspect    58 above, wherein the binder comprises an inorganic compound.-   [62] A coating agent according to aspect 61 above, wherein the    inorganic compound is selected from a group consisting of Zr    compounds, Si compounds, Ti compounds and Al compounds.-   [63] A method of producing a film which displays photocatalytic    properties by applying a coating agent and curing the thus produced    film, wherein the curing temperature is no more than 500° C., and    the coating agent utilizes a coating agent according to any one of    aspect 57 through aspect 62 above.-   [64] A method of producing a film which displays photocatalytic    properties according to aspect 63 above, wherein the curing    temperature is no more than 200° C.-   [65] A method of producing a film which displays photocatalytic    properties according to aspect 63 above, wherein the curing    temperature is no more than 30° C.-   [66] An article with a film which displays photocatalytic    properties, wherein the film which displays photocatalytic    properties is produced by a method according to any one of aspect 63    through aspect 66 above.-   [67] An article with a film which displays photocatalytic    properties, wherein when a film of surface area 400 cm² which    displays photocatalytic properties, placed within 5 L of dry air    containing 60 ppm by volume of hydrogen sulfide, is irradiated with    a day white fluorescent lamp producing an ultraviolet light    intensity of 6 μW/cm² at a wavelength of 365 nm, the ratio of    decomposition of the hydrogen sulfide after four hours of    irradiation is at least 20%.-   [68] An article according to either one of aspect 66 and aspect 67    above, wherein the film which displays photocatalytic properties has    a film thickness within a range from 0.01 to 100 μm.-   [69] An article according to aspect 68 above, wherein the film    thickness is from 0.01 to 0.1 μm.-   [70] An article according to aspect 68 above, wherein the film    thickness is from 1 to 100 μm.-   [71] An article according to either one of aspect 69 and aspect 70    above, wherein if the light transmittance at 550 nm for the article    without a film which displays photocatalytic properties is termed    T1%, and the light transmittance at 550 nm for the article with a    film which displays photocatalytic properties is termed T2%, then    the article has a film which displays photocatalytic properties with    a portion for which the ratio T2/T1 is at least 0.9.-   [72] An article according to either one of aspect 69 and aspect 70    above, wherein if the light reflectance at 550 nm for the article    without a film which displays photocatalytic properties is termed    R1%, and the light reflectance at 550 nm for the article with a film    which displays photocatalytic properties is termed R2%, then the    article has a film which displays photocatalytic properties with a    portion for which the ratio R2/R1 is at least 0.9.-   [73] An article according to any one of aspect 66 through aspect 72    above, wherein the film which displays photocatalytic properties has    a pencil hardness of at least 2H.-   [74] An article according to any one of aspect 66 through aspect 73    above, wherein after 24 hours irradiation with light from a day    white fluorescent lamp producing an ultraviolet light intensity of 6    μW/cm² at a wavelength of 365 nm, the film which displays    photocatalytic properties displays a contact angle with respect to    water of no more than 20°.-   [75] An article according to aspect 74 above, wherein the contact    angle with respect to water is no more than 10°.-   [76] An article according to aspect 75 above, wherein the contact    angle with respect to water is no more than 5°.-   [77] An article according to any one of aspect 66 through aspect 76    above, wherein after 24 hours irradiation with light from a day    white fluorescent lamp producing an ultraviolet light intensity of 6    μW/cm² at a wavelength of 365 nm, and subsequent storage for 24    hours in the dark, the film which displays photocatalytic properties    displays a contact angle with respect to water of no more than 20°.-   [78] An article according to aspect 77 above, wherein the contact    angle with respect to water after storage for 24 hours in the dark    is no more than 10°.-   [79] An article according to aspect 78 above, wherein the contact    angle with respect to water after storage for 24 hours in the dark    is no more than 5°.-   [80] An article according to any one of aspect 66 through aspect 79    above, wherein after a 4000 hour xenon arc lamp accelerated exposure    test, the film which displays photocatalytic properties displays a    degree of yellowing of no more than 10, and a contact angle with    respect to water of no more than 20° after 24 hours irradiation with    light from a day white fluorescent lamp producing an ultraviolet    light intensity of 6 μW/cm² at a wavelength of 365 nm.-   [81] An article according to any one of aspect 66 through aspect 80    above, wherein the film which displays photocatalytic properties is    formed on an inorganic substrate.-   [82] An article according to aspect 81 above, wherein the inorganic    substrate is a metal or a ceramic.-   [83] An article according to aspect 82 above, wherein the inorganic    substrate is at least one inorganic substrate selected from a group    consisting of Si compounds and Al compounds.-   [84] An article according to any one of aspect 66 through aspect 80    above, wherein the film which displays photocatalytic properties is    formed on an organic substrate.-   [85] An article according to aspect 84 above, wherein the organic    substrate is an organic polymer.-   [86] An article according to aspect 85 above, wherein the organic    polymer is at least one organic polymer selected from a group    consisting of polyethylene, polypropylene, polystyrene, nylon 6,    nylon 66, aramid, polyethylene terephthalate, unsaturated polyester,    polyvinyl chloride, polyvinylidene chloride, polyethylene oxide,    polyethylene glycol, silicon resin, polyvinyl alcohol, vinyl acetal    resin, polyacetate, ABS resin, epoxy resin, vinyl acetate resin,    cellulose, rayon and other cellulose derivatives, urethane resin,    polyurethane, urea resin, fluororesin, polyvinylidene fluoride,    phenol resin, celluloid, chitin, starch sheet, acrylic resin,    melamine resin and alkyd resin.-   [87] An article according to any one of aspect 81 through aspect 86    above, wherein the article is at least one article selected from a    group consisting of building materials, machinery, vehicles, glass    products, electric appliances, agricultural materials, electronic    equipment, tools, eating utensils, bath goods, toilet goods,    furniture, clothing, fabric products, fibers, leather goods, paper    products, sports goods, bedding, containers, spectacles, signboards,    piping, wiring, brackets, hygiene materials and automobile goods.-   [88] A method of imparting photocatalytic properties and    hydrophilicity, wherein the light source for generating    photocatalytic properties and hydrophilicity in an article according    to any one of aspect 47, aspect 48 and aspect 87 is at least one    light source selected from a group consisting of sun, fluorescent    lamps, mercury lamps, xenon lamps, halogen lamps, mercury xenon    lamps, metal halide lamps, light emitting diodes, lasers, and the    combustion flames from organic material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sample light intensity spectrum for a day white fluorescentlamp.

FIG. 2 is a schematic illustration of a reaction device for an example6.

FIG. 3 is an absorption spectrum for a photocatalytic slurry of anexample 1.

DETAILED DESCRIPTION OF THE INVENTION

A more detailed description of the present invention follows.

Among the methods of producing composite particles of titanium dioxideand a compound inactive as a photocatalyst according to the presentinvention, the methods described below are presented as preferredmethods. Basically, aggregation of the raw material titanium dioxide andthe generated composite particles is very strongly suppressed throughoutthe entire process, and as a result, novel composite particles with thetype of photocatalytic function and durability not obtainable withconventional surface treatment methods can be produced.

Specifically, the method of producing the composite particles comprisesthe steps of preparing a water based slurry of pH 3 to 5 comprisingtitanium dioxide, preparing a water based solution comprising a compoundinactive as a photocatalyst, and reacting the slurry and the solutiontogether at a pH within a range from 4 to 10. This method is describedin more detail below.

First, a water based slurry comprising titanium dioxide is prepared.

The BET specific surface area of the titanium dioxide is preferablywithin a range from 10 to 300 m²/g, and even more preferably from 30 to250 m²/g, and most preferably from 50 to 200 m²/g. If the BET specificsurface area is less than 10 m²/g then the photocatalytic functiondecreases, whereas at values exceeding 300 m²/g, productivitydeteriorates to an impractical level.

The crystal type of the titanium dioxide may be any of anatase, rutileand brookite crystals, although anatase or brookite crystals arepreferred, and brookite crystals are the most desirable. Furthermore,the titanium dioxide may also comprise at least two crystal types fromamong anatase, rutile and brookite crystals. If the titanium dioxidecomprises at least two crystal types, then in some cases the activityexceeds that of any of the singular crystal types.

There are no particular restrictions on the method of producing thetitanium dioxide, and suitable methods include vapor phase methods usingTiCl₄ as the raw material, and liquid phase methods using an aqueoussolution of TiCl₄ or an aqueous solution of titanyl sulfate as the rawmaterial.

An example of a vapor phase method is the method presented inInternational Patent Application, No. WO01/16027. Specifically, this isa method of producing ultra fine particles of titanium oxide with a BETspecific surface area of 10 to 200 m²/g, wherein a gas containingtitanium tetrachloride and an oxidizing gas are each preheated to atemperature of at least 500° C., and each gas is then supplied to areaction tube at a flow velocity of at least 10 m/s. Furthermore, theparticles containing titanium dioxide may also be ultra fine particlesof mixed crystal oxides incorporating mixed crystals withtitanium-oxygen-silicon bonds within the primary particles. An exampleof a method for producing such particles is the method presented inInternational Patent Application, No. WO01/56930. This is a method ofproducing ultra fine oxide particles containing mixed crystal primaryparticles with a BET specific surface area of 10 to 200 m²/g, wherein amixed gas comprising at least two halogenated metal compounds selectedfrom a group consisting of chlorides, bromides and iodides of titaniumand silicon (hereafter referred to as a “mixed halogenated metal gas”),and an oxidizing gas are each preheated to a temperature of at least500° C., and the two gases are then reacted together. In this method,both the mixed halogenated metal gas and the oxidizing gas arepreferably supplied to the reaction tube at a flow velocity of at least10 m/s, and even more preferably at least 30 m/s, and the gases arepreferably reacted together in the reaction tube at a high temperatureexceeding 600° C., with the residence and reaction time of the gaseswithin the tube being no more than 1 second.

An example of a liquid phase method is the method presented in JapaneseUnexamined Patent Application, Laid-open No. Hei 11-43327. This is amethod of producing a water dispersed sol of brookite titanium oxide byadding titanium tetrachloride to hot water of 75 to 100° C., andconducting a hydrolysis at a temperature within a range from 75° C. tothe boiling point of the solution. In order to impart a high degree oftransparency to a slurry, a coating agent and a film according to thepresent invention, this type of titanium dioxide synthesized by a liquidphase method is the preferred raw material. In addition, the liquidphase synthesized titanium dioxide is preferably used by maintaining theslurry state produced during the synthesis, that is, without passingthrough a step of producing a titanium dioxide powder. If a step forproducing a powder is employed after the liquid phase synthesis, thenaggregation of the titanium dioxide occurs, making it more difficult toachieve a high degree of transparency. Furthermore, although techniquesare available for breaking up such aggregations, using an air flowcrushing device such as a jet mill or a micronizer, a roller mill, or apulverizer, these techniques increase processing time, and lead tocontamination by impurities during the crushing step and a lack ofuniformity of particle size distribution, and are consequently notdesirable.

The titanium dioxide concentration of the prepared water based slurrycomprising titanium dioxide is preferably within a range from 0.1 to 10mass %. Concentration values from 0.5 to 5 mass % are even morepreferred. If the slurry concentration of titanium dioxide exceeds 10mass %, then the titanium dioxide undergoes undesirable aggregation inthe mixing step described below. Furthermore, if the concentration isless than 0.1 mass %, then the productivity deteriorates to anundesirable level.

The pH of the prepared water based slurry comprising titanium dioxide ispreferably within a range from 3 to 5. If the pH is lower than 3, thenduring the reaction step described below, localized neutralization andheat generation causes undesirable aggregation of the titanium dioxideduring the mixing process. Furthermore, if the pH exceeds 5, thenaggregation of the titanium dioxide is more likely to proceed, which isalso undesirable. After preparation of water based slurry of theaforementioned titanium dioxide from the vapor phase method or from theliquid phase method, the pH can be adjusted if necessary, usingtreatment by electrodialysis or an ion exchange resin.

Next, a water based solution comprising a compound inactive as aphotocatalyst is prepared. One possible method for complexing thecompound inactive as a photocatalyst with the aforementioned titaniumdioxide involves adding a powder of the compound inactive as aphotocatalyst to the titanium dioxide slurry described above, anddissolving the powder, but this method results in a reduction in thevisible light absorption coefficient of the titanium dioxide describedbelow, and is consequently undesirable.

Examples of the compound inactive as a photocatalyst include phosphates,condensed phosphates, borates, sulfates, condensed sulfates,carboxylates, Si compounds, Al compounds, P compounds, S compounds and Ncompounds. Furthermore, these compounds can be used singularly, or incombinations of a plurality of compounds. Of the above compounds, saltsof polybasic acids such as condensed phosphates, borates, condensedsulfates and polyvalent carboxylates are preferred, and condensedphosphates are particularly desirable.

Examples of condensed phosphates include pyrophosphates,tripolyphosphates, tetrapolyphosphates, metaphosphates andultraphosphates. Of these, pyrophosphates and tripolyphosphates areparticularly preferred.

The cations contained within the above salts are preferably alkalimetals, alkali earth metals, transition metals or Al. Amongst alkalimetals, Na and K are preferred. Amongst alkali earth metals, Mg and Caare preferred. Amongst transition metals, Fe and Zn are preferred.

Furthermore, in those case in which the compound inactive as aphotocatalyst for complexing with the titanium dioxide is only sparinglysoluble in water, aqueous solutions of a plurality of raw materials forgenerating the sparingly water-soluble compound are prepared. Forexample, in order to complex calcium pyrophosphate with titaniumdioxide, an aqueous solution of sodium pyrophosphate and an aqueoussolution of calcium chloride are prepared.

The concentration of the compound inactive as a photocatalyst(hereafter, this compound may also be referred to as an “inactivecompound”) within the water based solution comprising the inactivecompound is preferably no more than 40 mass %, and even more preferablyno more than 20 mass %. At concentration levels exceeding 40 mass %,undesirable localized aggregation of the titanium dioxide occurs uponmixing during the mixing step described below.

The total quantity of the prepared compound inactive as a photocatalystis typically within a range from 0.01 to 100 mass %, and preferably from0.1 to 50 mass %, based on the mass of titanium dioxide. If the totalquantity of the compound inactive as a photocatalyst is less than 0.01mass %, then the reactivity with titanium dioxide deteriorates. Incontrast, if the total quantity of the compound inactive as aphotocatalyst exceeds 50 mass %, then the process becomes uneconomic,and also leads to aggregation of the titanium dioxide.

Next, the water based slurry comprising titanium dioxide and the waterbased solution comprising a compound inactive as a photocatalyst aremixed together and reacted.

The pH at mixing is preferably within a range from 4 to 10, with pHvalues from 5 to 9 being even more desirable. If the pH is lower than 4,then the reactivity between the titanium dioxide and the compoundinactive as a photocatalyst is undesirably low. Furthermore if the pHexceeds 10, then aggregation of the titanium dioxide occurs upon mixing,which is also undesirable. In addition, in terms of selection of thematerial for the reaction apparatus, if the mixing pH is lower than 4,then cheap metal materials such as stainless steel can no longer beused.

In order to adjust the pH at mixing, either pH adjustment can beconducted at the time of mixing of the water based slurry comprisingtitanium dioxide and the water based solution comprising a compoundinactive as a photocatalyst, or the pH of the water based solutioncomprising a compound inactive as a photocatalyst can be adjusted inadvance so that the pH at mixing falls within a set range. Examples ofmethods for adjusting the pH include using aqueous solutions of mineralacids such as hydrochloric acid or sulfuric acid, or aqueous solutionsof sodium hydroxide or ammonia. However, in order to avoid localizedaggregation of the raw material titanium dioxide or the productcomposite particles within those regions where such a pH regulator hasbeen added and mixed, the quantity of the pH regulator should preferablybe kept to an absolute minimum, or only dilute concentrations should beused.

Examples of suitable methods for mixing the water based slurrycomprising titanium dioxide and the water based solution comprising acompound inactive as a photocatalyst include adding the water basedsolution comprising a compound inactive as a photocatalyst in acontinuous manner to the water based slurry comprising titanium dioxide,or adding both reactants simultaneously to a reaction vessel.

The concentration of titanium dioxide following mixing of the waterbased slurry comprising titanium dioxide and the water based solutioncomprising a compound inactive as a photocatalyst is preferably no morethan 5 mass %, and even more preferably no more than 3 mass %. If themixing process results in a post mixing concentration exceeding 5 mass%, then undesirable localized aggregation of the titanium dioxide mayoccur during mixing.

The temperature of the reaction between the water based slurrycomprising titanium dioxide and the water based solution comprising acompound inactive as a photocatalyst is preferably no more than 50° C.,and even more preferably no more than 30° C. If the temperature exceeds50° C., then undesirable aggregation of the fine particles occurs insidethe reaction vessel.

In addition, the water based slurry following reaction may also besubjected to demineralization. Removal of excess salts is effective inincreasing the dispersibility of the particles. Examples ofdemineralization methods include methods using ion exchange resins,methods using electrodialysis, methods using ultrafiltration membranes,and methods using a rotary filter press (manufactured by KotobukiEngineering and Manufacturing Co., Ltd.). The pH followingdemineralization is preferably within a range from 5 to 9, and even morepreferably from 6 to 8.

If a compound inactive as a photocatalyst is present on the surface oftitanium dioxide, then the photocatalytic activity will typicallydecrease, but surprisingly the inventors discovered that if surfacetreatment is performed using the method described above, then thephotocatalytic activity improves beyond that of the untreated product,even though a compound inactive as a photocatalyst is present on thesurface of the titanium dioxide. Furthermore, this effect is realizedwhen aggregation of the raw material titanium dioxide and the generatedcomposite particles is strongly suppressed throughout the entireprocess, as in the present invention. The effect is particularly markedin cases where the surface is partially treated with a polybasic acid.The reason for this observation is not entirely clear, although it isconsidered that one factor may be that-multi electron attractingcarboxyl groups or sulfonyl groups and the like display a preferredinteraction with specific Ti atoms on the surface of the titaniumdioxide, and as a result electrons generated within the titanium dioxideparticles upon light absorption undergo charge separation at thesurface, causing an increase in the photocatalytic activity.

Furthermore, it is thought that another factor is that new energy levelsare formed for composite oxides comprising specific Ti atoms on thesurface of the titanium dioxide, and that, depending on the nature ofthose composite oxides, band gaps which are responsive to visible lightcan arise. Generally, it is assumed that surface treatment with amaterial inactive as a photocatalyst leads to a suppression of thephotocatalytic activity of the titanium dioxide, but this is notnecessarily always true. Furthermore, another advantage is that becauseat least the terminal atomic groups of the surface treatment groups arephotocatalytically inactive, and the surface treatment groups inhibitcontact between organic based materials and the titanium oxide in asteric manner, then in those cases in which the particles are applied toorganic based materials, the durability of the organic based materialsimproves. Generally, it is thought that this phenomenon arises becausethe materials to undergo decomposition are either gases or liquids, andthe positional relationship between these materials and thephotocatalyst particles is fluid (that is, the materials to undergodecomposition are movable), whereas the organic substrate is a solid,and the steric positional relationship between the photocatalystparticles and the organic substrate is a fixed relationship.

In other words, only by using a surface treatment process in which thedispersibility of the titanium dioxide is maintained throughout, can aneffective interaction be realized between the polybasic acid andspecific Ti atoms, thereby enabling levels of photocatalytic activityand durability which exceed those of the raw material, and thegeneration of a slurry with superior dispersibility.

A description of the photocatalytic activity of the photocatalystparticles follows.

There are no particular restrictions on the method of measuring thephotocatalytic activity, and an example of a suitable method involvesspreading 3.5 g of the photocatalyst particles uniformly across a flatsurface with a diameter of 9 cm, placing the sample in 5 L of dry aircontaining 20 ppm by volume of acetaldehyde, irradiating the sample witha day white fluorescent lamp producing an ultraviolet light intensity of6 μW/cm² at a wavelength of 365 nm, and then determining the ratio ofdecomposition of the acetaldehyde (hereafter, may be abbreviated as DWA)after one hour of irradiation.

This decomposition ratio can be measured in the manner described below,for example. A glass laboratory dish having an internal diameter of 9 cmcontaining 3.5 g of photocatalyst particles (or a powder containing suchparticles) spread evenly across the bottom surface of the dish is placedinside a 5 L capacity vessel with good transmittance of visible lightthrough to ultraviolet light (such as a bag formed from polyvinylfluoride film). Subsequently, 5 L of dry air containing 20 ppm by volumeof acetaldehyde is charged into and removed from the vessel at leastonce, and a further 5 L of dry air containing 20 ppm by volume ofacetaldehyde is used to fill the vessel again, ensuring the gas insidethe vessel has been adequately exchanged. The vessel is then irradiatedfrom the exterior for a period of one hour, and the ratio ofdecomposition, excluding any adsorption of the acetaldehyde (hereafter,this value is simply described as the “ratio of decomposition”), ismeasured. In this measurement, a day white fluorescent lamp is used asthe light source, resulting in light with an ultraviolet light intensityof 6 μW/cm² at a wavelength of 365 nm being irradiated onto the spreadout photocatalyst particles.

This measurement is described below in further detail.

In those cases in which the particles are in the form of a powder, thepowder is first prepared. In those cases in which the particles are inthe form of a slurry, the slurry is dried, either by heating or underreduced pressure, preferably at a temperature exceeding the boilingpoint of the solvent, and a crushed powder is prepared. In the case of awater based slurry, the drying should be conducted at a temperature of100 to 120° C. 3.5 g of powder prepared in this manner is spread evenlyacross the bottom surface of a glass laboratory dish with an internaldiameter of 9 cm, and the dish is then placed inside a 5 L capacity bagmade of polyvinyl fluoride film. An example of a suitable polyvinylfluoride film is a Tedlar Bag (AAK-5, manufactured by GL Sciences Inc.).Meanwhile, dry air containing 20 ppm by volume of acetaldehyde can beprepared from dry air using a permeator (PD-1B, manufactured by GasTechCorporation). The dry air can utilize commercially available compressedair (such as air compressed to a pressure of approximately 14.7 MPa at35° C., with any condensed water and compressor oil removed).Subsequently, 5 L of dry air containing 20 ppm by volume of acetaldehydeis charged into and removed from the polyvinyl fluoride film bag atleast once. The titanium dioxide will adsorb a certain quantity of theacetaldehyde, and consequently this operation is necessary. 5 L of gasof the same concentration is then used to refill the bag again, and theinitial acetaldehyde concentration C0T (ppm by volume) in the bag ismeasured using a detector tube (No. 92L, manufactured by GasTechCorporation).

The initial acetaldehyde concentration at measurement is preferably nomore than 50 ppm by volume, and even more preferably no more than 20 ppmby volume. In order to evaluate the deodorizing effect within livingenvironment spaces, extremely low concentration conditions arepreferred. For example, if acetaldehyde concentration is greater than1.4 ppm by volume, then it is detected as a strong odor. Furthermore,even if measurements are conducted at concentration levels exceeding 100ppm by volume, the results will not necessarily indicate photocatalyticfunction at low concentration levels. This phenomenon can also beappreciated through a Langmuir Hinshelwood interpretation in catalyticreaction rate analysis.

A day white fluorescent lamp is prepared as a light source. Examples ofsuitable day white fluorescent lamps include “Hi-white” FL20SS-N/18-B,manufactured by Hitachi GE Lighting Co., Ltd. The relative energyspectrum of this type of fluorescent lamp is known, and is shown in thespectrum of FIG. 1 (day white fluorescent lamp catalog, Hitachi GELighting Co., Ltd.).

Measurement of the light intensity utilizes, for example, a UVA-365device, manufactured by Atex Co., Ltd. Using this device, the lightintensity at 365 nm can be measured.

Next, light irradiation is commenced from outside the bag using a lightof predetermined light intensity. One hour after this irradiationcommencement point, the acetaldehyde concentration C1T in the bag (ppmby volume) is remeasured.

As a control test, a test is also conducted using the same operations asabove, but placing the bag in the dark for 1 hour. The initialacetaldehyde concentration for this control test is termed C0B (ppm byvolume), and the acetaldehyde concentration after 1 hour is termed C1B(ppm by volume).

The rate of decomposition excluding adsorption (DWA) is defined as:DWA={(C0T−C1T)−(C0B−C1B)}/C0T×100(%)

Photocatalyst particles according to the present invention are compositeparticles of titanium dioxide and a compound inactive as aphotocatalyst, and display a higher photocatalytic activity than the rawmaterial titanium dioxide particles. Specifically, the DWA value of thecomposite particles is higher than the DWA value for the raw materialtitanium dioxide particles.

Furthermore, photocatalyst particles according to the present inventioncomprise composite particles of titanium dioxide and a compound inactiveas a photocatalyst, wherein when 3.5 g of the photocatalyst particlesspread uniformly across a flat surface of a diameter of 9 cm, placed in5 L of dry air containing 20 ppm by volume of acetaldehyde, isirradiated with a day white fluorescent lamp producing an ultravioletlight intensity of 6 μW/cm² at a wavelength of 365 nm, the ratio ofdecomposition of the acetaldehyde after one hour of irradiation is atleast 20%. The DWA value is preferably at least 40%, and even morepreferably 80% or greater.

The BET specific surface area of the photocatalyst particles ispreferably within a range from 10 to 300 m²/g, and even more preferablyfrom 30 to 250 m²/g, and most preferably from 50 to 200 m²/g. If the BETspecific surface area is less than 10 m²/g then the photocatalyticfunction decreases, whereas at values exceeding 300 m²/g, productivitydeteriorates to an impractical level.

The crystal type of the titanium dioxide incorporated within thephotocatalyst particles may be any of anatase, rutile and brookitecrystals, although anatase or brookite crystals are preferred, andbrookite crystals are the most desirable. Furthermore, the titaniumdioxide may also comprise at least two crystal types from among anatase,rutile and brookite crystals. If the titanium dioxide comprises at leasttwo crystal types, then in some cases the activity exceeds that of anyof the singular crystal types.

Furthermore, the compound inactive as a photocatalyst may exist insidethe titanium dioxide particles or on the surface of the particles. Inthose cases in which the compound is present on the surface of thetitanium dioxide particles, it is preferable that the compound form apartial surface coating. In the former case, n-type or p-typesemiconductors can be formed, improving the visible light activity,whereas in the latter case, by suppressing contact with organic matter,the number of fields of potential practical application for theparticles can be increased.

Next, a description is given of the latter of the two cases describedabove. Examples of the compound inactive as a photocatalyst includephosphates, condensed phosphates, borates, sulfates, condensed sulfates,carboxylates, Si compounds, Al compounds, P compounds, S compounds and Ncompounds. Other examples include silica, zirconia, alumina, magnesia,calcia, amorphous titania, mullite and spinel. These compounds can beused singularly, or in combinations of a plurality of compounds.

Of the above compounds, salts of polybasic acids such as condensedphosphates, borates, condensed sulfates and polyvalent carboxylates arepreferred, and condensed phosphates are particularly desirable.

Examples of condensed phosphates include pyrophosphates,tripolyphosphates, tetrapolyphosphates, metaphosphates andultraphosphates. Of these, pyrophosphates and tripolyphosphates areparticularly preferred.

The cations contained within the above salts are preferably alkalimetals, alkali earth metals, transition metals or Al. Amongst alkalimetals, Na and K are preferred. Amongst alkali earth metals, Mg and Caare preferred. Amongst transition metals, Fe and Zn are preferred.

The quantity of these types of compounds inactive as a photocatalyst istypically within a range from 0.01 to 50 mass %, and preferably from 0.1to 20 mass %, based on the mass of titanium dioxide. If the quantity ofthe compound inactive as a photocatalyst is less than 0.01 mass %, thenthe photocatalytic activity of the titanium dioxide with respect tomedia such as plastic, paper or fiber causes a deterioration in thedurability of the medium itself. In contrast, if the quantity of thecompound inactive as a photocatalyst exceeds 50 mass %, then thephotocatalyst particles become uneconomic.

An example of a preferred embodiment comprises composite particles oftitanium dioxide and a condensed phosphate, wherein if the BET specificsurface area is A m²/g and the quantity of P within the particles is Bmass %, then A≧50 and B/A is within a range from 0.002 to 0.01.Particles in which a condensed phosphate is present on the surface of abrookite titanium dioxide or an anatase titanium dioxide are even moredesirable.

In addition, the isoelectric point determined from the zeta potentialmeasured for the photocatalyst particles of the present invention usingan electrophoresis light scattering method is preferably no more than 4,and even more preferably no more than 3, and most preferably no morethan 2. A description of a method of measuring the zeta potential isprovided below.

Many different methods exist for measuring the zeta potential, but themeasurement principle adopted in the present invention involvesanalyzing the electrophoretic rate from the size of the frequency shiftaccording,to a laser Doppler method, or a so-called electrophoresislight scattering method. Specifically, the zeta potential can bemeasured using a ELS-8000 device, manufactured by Otsuka ElectronicsCo., Ltd.

0.01 g (the tip of a spatula) of a sample powder is placed in 50 ml of a0.01 mol/l NaCl solution, the pH is adjusted, where necessary, by adding0.01 or 0.1 mol/l solutions of HCl or NaOH, and the mixture is subjectedto ultrasound dispersion for approximately one minute before beingshifted to the measurement device.

A photocatalytic powder of the present invention can be added to anorganic polymer and used as a composition. Examples of organic polymerswhich can be used include thermoplastic resins, thermosetting resins andnatural resins. Because of the presence of the aforementioned compoundinactive as a photocatalyst, the organic polymer and thephotocatalytically active surface of the titanium dioxide do not come indirect contact, and consequently the organic polymer of the medium doesnot deteriorate through decomposition, meaning the durability of theorganic polymer can be improved.

Specific examples of this type of organic polymer include polyolefinssuch as polyethylene, polypropylene and polystyrene; polyamides such asnylon 6, nylon 66 and aramid; polyesters such as polyethyleneterephthalate and unsaturated polyester; as well as polyvinyl chloride,polyvinylidene chloride, polyethylene oxide, polyethylene glycol,silicon resin, polyvinyl alcohol, vinyl acetal resin, polyacetate, ABSresin, epoxy resin, vinyl acetate resin, cellulose, rayon and othercellulose derivatives, urethane resin, polyurethane, urea resin,fluororesin, polyvinylidene fluoride, phenol resin, celluloid, chitin,starch sheet, acrylic resin, melamine resin and alkyd resin.

These types of organic polymer compositions incorporating aphotocatalytic powder of the present invention can be used in a varietyof forms including compounds or master batches. The concentration ofphotocatalytic powder within such all organic polymer composition ispreferably within a range from 0.01 to 80 mass %, and even morepreferably from 1 to 50 mass %, based on the total mass of thecomposition. Furthermore, in order to improve the removal of substanceswith offensive odors, adsorbents such as active carbon or zeolite may beadded to the organic polymer composition. In the present invention, apolymer molded article with photocatalytic properties can be produced bymolding a polymer composition described above. Examples of this type ofmolded article include fiber, film and plastic molded articles.Specifically, these types of molded articles can be applied to a varietyof building materials, machinery, vehicles, glass products, electricappliances, agricultural materials, electronic equipment, tools, eatingutensils, bath goods, toilet goods, furniture, clothing, fabricproducts, fibers, leather goods, paper products, sports goods, bedding,containers, spectacles, signboards, piping, wiring, brackets, hygienematerials, automobile goods, outdoor goods such as tents, masks,stockings and socks.

A slurry in the present invention describes a solvent dispersion of theaforementioned photocatalyst particles. There are no particularrestrictions on the method of preparing the slurry, and suitable methodsinclude diluting the slurry with a solvent following the aforementionedsurface treatment reaction, and methods in which the surface treatmentreaction slurry is filtered and washed to yield a solid containing thephotocatalyst particles, and a solvent is then added to this solid. Inthe latter case, aggregation of the particles may occur, so the formermethod is preferred.

There are no particular restrictions on the solvent used in theformation of the slurry, although because the surface of thephotocatalyst particles is typically hydrophilic, hydrophilic solventsare preferred. Water based solvents such as water or mixed solvents ofwater and hydrophilic organic solvents are particularly preferred.

There are no particular restrictions on the content of photocatalystparticles within the slurry, although quantities within a range from0.01 to 50 mass %, and more preferably from 1 to 40 mass % aredesirable. If the quantity of photocatalytic powder is less than 0.01mass % then an adequate photocatalytic effect cannot be achieved aftercoating. In contrast, if the quantity of powder exceeds 50 mass %, thennot only do problems of increased viscosity arise, but the coating alsobecomes uneconomic.

Furthermore, when a solvent comprising water is used, the pH of theslurry is preferably within a range from 5 to 9, and even morepreferably from 6 to 8. If the pH is lower than 5 or greater than 9,then the slurry has an undesirable effect on living organisms and theenvironment, and displays a corrosive action towards metals which cannotbe ignored, making the slurry unsuitable for use on metal substrates.

In addition, a slurry of the present invention is characterized by ahigh transmittance value. A description of a method of measuring thetransmittance follows. A spectrophotometer or a spectrophotometriccalorimeter is used for measuring the transmittance. The descriptionbelow focuses on measurements using a CM-3700d spectrophotometriccalorimeter manufactured by Minolta Co., Ltd.

First a slurry of 10% concentration is prepared in a glass cell with anoptical path length of 2 mm. Using a xenon lamp as the light source,diffuse reflected light from an integrating sphere is irradiated throughthe sample inside the glass cell, and the transmitted light is capturedby a measuring spectrometer. Meanwhile, diffused light from inside theintegrating sphere is captured by an illuminating light spectrometer,and each light sample is dispersed into a spectrum, and thetransmittance is measured at each wavelength.

A characteristic of the present invention is that when the concentrationof photocatalyst particles within the slurry is set to 10 mass %, thetransmittance at 550 nm through a 2 mm thickness of slurry (the opticalpath length) is at least 20%, and preferably at least 30%. By using thistype of slurry, the design and coloring of the object to be coated canbe largely retained, making the slurry very advantageous in practicalapplications.

Furthermore, a slurry of the present invention also has good visiblelight absorption characteristics across a wide portion of the visiblelight spectrum.

Here, the absorption coefficient is defined as:Absorption coefficient=100−transmittance−reflectance  (A)

In the formula (A), the transmittance is the value measured according tothe method described above.

Measurement of the reflectance in the formula (A) can utilize the sameapparatus used for measuring the transmittance. First, a similar sampleto that used for measuring the transmittance (a slurry of 10%concentration placed in a glass cell with an optical path length of 2mm) is prepared. Using a xenon lamp as the light source, diffusereflected light from an integrating sphere is irradiated onto the sampleinside the glass cell, and of the reflected light, reflected light in adirection forming an angle of 8 degrees relative to the axisperpendicular to the sample surface is captured by a measuringspectrometer. Meanwhile, diffused light from inside the integratingsphere is captured by an illuminating light spectrometer, and each lightsample is dispersed into a spectrum, and the reflectance is measured ateach wavelength.

A characteristic of the present invention is that when the concentrationof photocatalyst particles within the slurry is set to 10 mass %, theabsorption coefficient at 400 nm for a 2 mm thickness of slurry (theoptical path length) is at least 25%, and preferably at least 30%.Furthermore, the absorption coefficient at 550 nm is preferably within arange from 8 to 30%, and even more preferably from 10 to 20%. If theabsorption coefficient at 550 nm is less than 8%, then visible lightcannot be used effectively, whereas if the absorption coefficientexceeds 30% then coloring becomes more noticeable.

In addition, in order to improve the adhesion and the photocatalyticfunction of the slurry during coating or molding, a variety of metaloxides may also be added to the slurry. The metal elements can beappropriately selected from the transition metals, alkali earth metals,alkali metals, group IIIb metals or group IV metals. Of these, Zr, Si,Sn and Ti are preferred, and Zr and Si are particularly desirable.

There are no particular restrictions on the method of adding a metaloxide, and in one suitable method, a sol synthesized by a liquid phasemethod using a metal alkoxide as the raw material is added to theslurry. In this case, the BET specific surface area of the metal oxideparticles is preferably within a range from 10 to 500 m²/g, and evenmore preferably from 30 to 450 m²/g, and most preferably from 50 to 400m²/g.

In addition, a method in which a metal alkoxide is added to the slurryand hydrolyzed, thereby depositing metal oxide particles onto thesurface of the photocatalyst particles, is also preferred. In this case,the metal oxide preferably forms a partial surface coating on thephotocatalyst particles. This partial coating may form islands, anarckipelago or a muskmelon pattern.

The reasons why addition of this type of metal oxide lead to animprovement in the photocatalytic function of a molded from such as acoating film are not entirely evident, although it is thought thatpossible reasons may include a promotion of charge separation on thephotocatalyst particles due to the electron attracting properties of theadded metal oxide, and the fact that in those cases where the conductionband level of the added metal oxide is lower than the conduction bandlevel of the photocatalyst paiticles, the metal oxide may captureelectrons from the photocatalyst particles.

Furthermore, a binder can be added to the dispersion (slurry) to form acoating agent, and a photocatalytic structural body can then be producedby applying the coating agent to the surface of the types of structuralbodies described below. In other words, the coating agent can be used asa paint or a coating composition. There are no particular restrictionson the binder material used in the present invention, and either anorganic based binder or an inorganic binder can be used. Suitableorganic binders include water soluble binders, and specific examplesinclude polyvinyl alcohol, melamine resin, urethane resin, celluloid,chitin, starch sheet, polyacrylamide and acrylamide and the like.Furthermore, suitable inorganic binders include Zr compounds, Sicompounds, Ti compounds and Al compounds, and specific examples includezirconium compounds such as zirconium oxychloride, zirconiumhydroxychloride, zirconium nitrate, zirconium sulfate, zirconiumacetate, zirconium ammonium carbonate and zirconium propionate; siliconcompounds such as alkoxysilanes, partial hydrolysis products ofalkoxysilanes produced using a mineral acid, and silicates; as well asmetal alkoxides of aluminum, Ti or zirconium, and partial hydrolysisproducts thereof produced using a mineral acid. Furthermore, othersuitable examples include materials produced by selecting a plurality ofmetal alkoxides from aluminum, silicon, titanium and zirconiumalkoxides, and complexing or hydrolyzing the mixture. Of these,cohydrolysis products of aluminum alkoxide and titanium alkoxide, andcohydrolysis products of aluminum alkoxide and silicon alkoxide arepreferred.

In particular, if a binder with a plurality of carboxyl groups orsulfonyl groups as functional groups is used, then the photocatalyticfunction under a practical, low intensity light source such as afluorescent lamp can be improved. A specific example of this type ofbinder is a water soluble urethane emulsion. The reason for thisimproved photocatalytic function is not entirely clear, although it isbelieved that possible factors include the fact that, in the same manneras described above in relation to the surface treatment of titaniumdioxide with a polybasic acid, the multi-electron attracting carboxylgroups or sulfonyl groups within the water soluble urethane emulsiondisplay an interaction with Ti atoms on the surface of the titaniumdioxide, and as a result, electrons generated within the titaniumdioxide particles upon light absorption undergo charge separation at thesurface, causing an increase in the photocatalytic activity, or thepossibility of a variation in the band gap at the surface of thetitanium dioxide.

The quantity of binder added to a coating agent is typically within arange from 0.01 to 20 mass %, and preferably from 1 to 10 mass %. If thequantity of binder is less than 0.01 mass %, then insufficient adhesionis produced following application, whereas if the quantity exceeds 20mass %, then not only do problems of increased viscosity arise, but thecoating also becomes uneconomic.

Furthermore, the pH of the coating agent following mixing with thebinder is preferably within a range from 5 to 9, and even morepreferably from 6 to 8. If the pH is lower than 5 or greater than 9,then the coating agent has an undesirable effect on living organisms andthe environment, and displays a corrosive action towards metals whichcannot be ignored, making the coating agent unsuitable for use on metalsubstrates. Depending on the pH level, the pH of the slurry comprisingthe photocatalyst particles may be adjusted in advance, in order toachieve a pH following mixing of the binder which falls within the rangefrom 5 to 9.

If either an organic binder or a partial hydrolysis product of analkoxysilane produced using a mineral acid is employed as the binder,then application and subsequent curing can be performed at temperaturesbelow 30° C. Furthermore, application can be performed at a temperaturebelow 30° C., and curing can then be performed at a temperature of nomore than 200° C. In addition, an inorganic binder can also be used onan inorganic substrate, with application performed at a temperaturebelow 30° C., and curing can then be performed at a temperature of nomore than 500° C., thereby generating a film with a high degree ofhardness. The photocatalytic function may also be increased by improvingthe crystallinity of the titanium dioxide within the film, and in somecases, heating at a temperature of 300 to 500° C. may be recommended.

Furthermore, the photocatalytic capacity of articles with a film whichdisplays photocatalytic activity according to the present invention havethe characteristics described below.

When a film of surface area 400 cm² which displays photocatalyticactivity, placed in 5 L of dry air containing 60 ppm by volume ofhydrogen sulfide, is irradiated with a day white fluorescent lampproducing an ultraviolet light intensity of 6 μW/cm² at a wavelength of365 nm, the ratio of decomposition of the hydrogen sulfide (hereafter,may be abbreviated as simply DWH) after six hours of irradiation, is atleast 20%.

This decomposition ratio can be measured in the manner described below,for example. An article with a film which displays photocatalyticproperties is placed inside a 5 L capacity bag formed from polyvinylfluoride film, so that the surface area of the article exposed to thesubsequent irradiation is 400 cm². Subsequently, 5 L of dry aircontaining 60 ppm by volume of hydrogen sulfide is charged into andremoved from the vessel at least once, and a further 5 L of dry aircontaining the same concentration of acetaldehyde is then used to fillthe vessel again, ensuring the gas inside the vessel has been adequatelyreplaced. The vessel is then irradiated from the exterior for a periodof six hours, and the ratio of decomposition, excluding any adsorptionof the hydrogen sulfide, is measured. In this measurement, a day whitefluorescent lamp is used as the light source, resulting in light with anultraviolet light intensity of 6 μW/cm² at a wavelength of 365 nm beingirradiated onto the article with a photocatalytic film.

In addition, because the present invention uses a slurry with a highlight transmittance value as the raw material, a film obtained from acoating agent using such a slurry as a raw material also displays a highdegree of transparency. In order to achieve a highly transparent film,it is recommended that titanium dioxide synthesized by a liquid phasemethod be used as the raw material for the photocatalyst particles.Specific examples of suitable photocatalyst particles include thoseproduced by the surface treatment, with a salt of a polybasic acid, ofparticles generated by thermal hydrolysis or neutral hydrolysis of anaqueous solution of TiCl₄ or an aqueous. solution of titanyl sulfate.Typically, the ideal thickness of a film for effectively exhibitingphotocatalytic activity is within a range from 0.01 to 100 μm.Furthermore, in order to effectively suppress interference fringes, thefilm thickness is preferably either within a range from 0.01 to 0.1 μm,or greater than 1 μm.

Provided the substrate is transparent, the transparency of thephotocatalytic film formed on top of the substrate can be described inthe following terms. If the light transmittance at 550 nm for thearticle without a film which displays photocatalytic properties (thatis, prior to film formation) is termed T1%, and the light transmittanceat 550 nm for the article with a film which displays photocatalyticproperties (that is, after film formation) is termed T2%, then acharacteristic of the present invention is a ratio T2/T1 which is atleast 0.9, and preferably at least 0.95. If the T2/T1 ratio is less than0.9, then the non-transparency for the substrate becomes noticeable.

In contrast, if the substrate is non-transparent, then the transparencyof the film formed on top of the substrate can be described in thefollowing manner, using the reflectance.

A spectrophotometer or a spectrophotometric colorimeter is used formeasuring the reflectance. The description below focuses on measurementsusing a CM-3700d spectrophotometric colorimeter manufactured by MinoltaCo., Ltd. Using a xenon lamp as the light source, diffuse reflectedlight from an integrating sphere is irradiated onto a film sample, andof the reflected light from the film, reflected light in a directionforming an angle of 8 degrees relative to the axis perpendicular to thesample surface is captured by a measuring spectrometer. Meanwhile,diffused light from inside the integrating sphere is captured by anilluminating light spectrometer, and each light sample is dispersed intoa spectrum, and the reflectance is measured at each wavelength.

If the light reflectance at 550 nm for the article prior to formation ofa film which displays photocatalytic properties is termed R1%, mid thelight reflectance at 550 nm for the article with a film which displaysphotocatalytic properties is termed R2%, then a characteristic of thepresent invention is a ratio R2/R1 which is at least 0.9, and preferablyat least 0.95. If the R2/R1 ratio is less than 0.9, then concealment andnon-transparency for the substrate become noticeable.

Furthermore another characteristic of the present invention is a filmwhich displays photocatalytic properties with a pencil hardness of atleast 2H. A film with a high pencil hardness means the film is moreresistant to scratches and marking. If a Zr compound is used as thebinder then a particularly strong film can be obtained with relativeease. There are no particular restrictions on the substrate (article),and both inorganic substrates and organic substrates are possible.Examples of inorganic substrates include Si compounds, Al compounds,various ceramics and metals. Specific examples include silica, alumina,mullite, spinel, zirconia, titania, graphite, carbon nanotube, diamond,iron, stainless steel, titanium, zircon, niobium and tantalum and thelike. Examples of suitable organic substrates include organic polymers.Specifically, a polymer can be selected from a group comprisingpolyethylene, polypropylene, polystyrene, nylon 6, nylon 66, aramid,polyethylene terephthalate, unsaturated polyester, polyvinyl chloride,polyvinylidene chloride, polyethylene oxide, polyethylene glycol,silicon resin, polyvinyl alcohol, vinyl acetal resin, polyacetate, ABSresin, epoxy resin, vinyl acetate resin, cellulose, rayon and othercellulose derivatives, urethane resin, polyurethane, urea resin,fluororesin, polyvinylidene fluoride, phenol resin, celluloid, chitin,starch sheet, acrylic resin, melamine resin and alkyd resin.

In addition, articles prepared from an aforementioned organic polymercomposition via a master batch or a compound state, or articles with afilm which displays photocatalytic properties, prepared via anaforementioned coating agent, formed on the surface thereof, can alsodisplay hydrophilicity. In order to achieve photocatalyst particleswhich exhibit effective hydrophilicity, it is recommended that titaniumdioxide synthesized by a liquid phase method be used as the rawmaterial. Specific examples of recommended photocatalyst particlesinclude those produced by the surface treatment, with a salt of apolybasic acid, of particles generated by thermal hydrolysis or neutralhydrolysis of an aqueous solution of TiCl₄ or an aqueous solution oftitanyl sulfate. The contact angle with respect to water can be used asan indicator of the hydrophilicity. A method of measuring the contactangle with respect to water is described below.

A droplet of pure water is transferred onto the film, and the contactangle between the film surface and the liquid droplet is measured. Thismeasurement is performed using a CA-D contact angle meter manufacturedby Kyowa Interface Science Co., Ltd. Using the syringe on the apparatus,a pure water droplet equivalent to 20 graduations on the syringe(diameter 1.5) is transferred gently to the film surface, and using anangle plate and a moveable reticule within an optical mirror, the peakof the liquid droplet is determined diagrammatically, the angle betweenthe line connecting the peak and the edge point of the liquid dropletand the film surface is read off directly, and that angle is doubled todetermine the contact angle.

In the present invention, hydrophilicity of a film which displaysphotocatalytic activity describes a film for which, after 24 hoursirradiation with light from a day white fluorescent lamp producing anultraviolet light intensity of 6 μW/cm² at a wavelength of 365 nm, thecontact angle with respect to water (hereafter, may be abbreviated asCL) is no more than 20°, and preferably no more than 10°, and even morepreferably no more than 5°.

Furthermore, excellent effects can also be achieved for the maintenanceof the hydrophilicity when a film is placed in the dark afterirradiation with light. Specifically, after 24 hours irradiation withlight from a day white fluorescent lamp producing an ultraviolet lightintensity of 6 μW/cm² at a wavelength of 365 nm, and subsequent storagefor 24 hours in the dark, a film which displays photocatalyticproperties according to the present invention displays a contact anglewith respect to water (hereafter, this contact angle may be abbreviatedas CD) of no more than 20°, and preferably no more than 10°, and evenmore preferably no more than 5°.

In this manner, by imparting hydrophilicity to the film surface, adhereddirt on the surface can be easily removed, and the clean surface can bemaintained over a long period of time, and can be readily restored ifrequired.

In addition, a film which displays photocatalyst properties according tothe present invention can also display good weather resistance.Specifically, when a film which displays photocatalytic properties issubjected to a xenon arc lamp accelerated exposure test (using aSUNSHINE XENON LONG LIFE WEATHER METER, manufactured by Suga TestInstruments Co., Ltd., BP temperature: 63±3° C., rainfall: 12/60minutes), then, even after 4000 hours, films can be produced for whichthe contact angle with respect to water following 24 hours irradiationwith light from a day white fluorescent lamp producing an ultravioletlight intensity of 6 μW/cm² at a wavelength of 365 nm is no more than20°, and the degree of yellowing is no more than 10.

There are no particular restrictions on the types of articles to whichthe photocatalytic properties and hydrophilicity described above can beimparted, and suitable examples include building materials, machinery,vehicles, glass products, electric appliances, agricultural materials,electronic equipment, tools, eating utensils, bath goods, toilet goods,furniture, clothing, fabric products, fibers, leather goods, paperproducts, sports goods, bedding, containers, spectacles, signboards,piping, wiring, brackets, hygiene materials and automobile goods.Furthermore, the invention can also be applied to environmental cleaningequipment and devices effective for treating sick house syndrome,decomposing organochlorine compounds such as PCBs or dioxin found inwater, the atmosphere or in soil, and decomposing residual agriculturalchemicals or environmental hormones in water or soil.

Furthermore, examples of suitable light sources for effectivelygenerating photocatalytic properties or hydrophilicity within the abovetype of articles include the sun, fluorescent lamps, incandescent lamps,mercury lamps, xenon lamps, halogen lamps, mercury xenon lamps, metalhalide lamps, light emitting diodes, lasers, and the combustion flamesfrom organic material. Furthermore, specific examples of fluorescentlamps include white fluorescent lamps, day white fluorescent lamps,daylight fluorescent lamps, warm white fluorescent lamps,incandescent-lamp-colored fluorescent lamps and black lights.

EXAMPLES

A description of specific features of the present invention, based on aseries of examples, follows, although the present invention is in no wayrestricted to the examples presented here.

Example 1

50 liters (hereafter, “liters” is abbreviated as “L”) of pure water wasmeasured, placed in a vessel and heated with stirring until thetemperature reached a steady 98° C. 3.6 kg of an aqueous solution oftitanium tetrachloride with a Ti concentration of 15 mass %(manufactured by Sumitomo Titanium Corporation) was then added dropwiseto the water over a period of 120 minutes. The white colored suspensionobtained on completion of the dropwise addition was then subjected todechlorination in an electrodialysis device, producing a slurry of pH 4.A sample was taken from the thus formed photocatalyst slurry, andmeasurement of the solid fraction concentration by drying to a constantweight revealed a value of 2 mass %. Structural analysis of the driedpowder using an X-ray diffraction apparatus revealed that the productpowder was a brookite titanium dioxide. The brookite content was 89 mass% and the anatase content was 11 mass %. The DWA value for the powderwas 11%.

Next, 100 g of sodium pyrophosphate (a food additive, manufactured byTaihei Chemical Industrial Co., Ltd.) was dissolved in pure water,forming 2 kg of a 5 mass % aqueous solution of sodium pyrophosphate.

50 L of the 2 mass % titanium dioxide slurry obtained above was placedin a reaction vessel and stirred well while cooling. 2 kg of the 5 mass% aqueous solution of sodium pyrophosphate and a 10 mass % aqueoussolution of caustic soda were added to the slurry over a period of onehour, to produce a mixed slurry with a pH within a range from 8 to 9.During the addition, the reaction temperature was maintained within arange from 20 to 25° C.

The thus produced pyrophosphate containing titanium dioxide slurry wasstored for 1 hour at a temperature of 22 to 28° C. The electricconductivity at this point was 10,000 μS/cm. Next, the slurry wasfiltered and cleaned using a rotary filter press (manufactured byKotobuki Engineering and Manufacturing Co., Ltd.), was then washed wellwith water until the electric conductivity of the filtrate reached 50μS/cm, and subsequently concentrated to produce a photocatalytic slurry.Measurement of the pH of the thus obtained photocatalytic slurry (usinga D-22 device manufactured by Horiba Ltd.) revealed a value of 7.8.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. to produce a powder. Calculation of the solid fractionconcentration of the slurry based on the quantity of powder revealed avalue of 10 mass %. Furthermore, the transmittance at 550 nm of a slurrysample of thickness 2 mm was 46%, and the slurry displayed excellentdispersibility. In addition, the absorption coefficient of a slurrysample of thickness 2 mm was 32% at 400 nm, and was 11% at 550 nm. Theabsorption coefficient spectrum is shown in FIG. 3. Analysis ofthepowder produced above using FT-IR (an FT-IR1650 apparatus, manufacturedby PerkinElmer Inc.) revealed pyrophosphate absorption. Next, analysisof the dried powder by ICP (using an ICPS-100V device, manufactured byShimadzu Corporation) revealed the existence of 0.7 mass % of Na and 1.2mass % of phosphorus. When the zeta potential was measured via anelectrophoresis light scattering method using a ELS-8000 device,manufactured by Otsuka Electronics Co., Ltd., the isoelectric point was2.1. Measurement of the BET specific surface area (using a Flow Sorb II2300 apparatus, manufactured by Shimadzu Corporation) produced a resultof 140 m²/g. Furthermore, the DWA value of the powder was 83%. Thisvalue is greater than the DWA value of the raw material titaniumdioxide, indicating that the surface treated product offers a higherlevel of photocatalytic activity.

(Preparation of a High Density Polyethylene Master Batch)

A portion of a photocatalytic slurry prepared in the same manner asdescribed above was dried using a media fluidized drying apparatus (aSLURRY DRYER, manufactured by Okawara Manufacturing Co., Ltd.), yielding5 kg of a photocatalytic powder comprising fine particles of titaniumdioxide with a condensed phosphate formed on the surface thereof. 20parts by mass of this photocatalytic powder, 2 parts by mass of zincstearate (ZINC STEARATE S, manufactured by NOF Corporation), and 78parts by mass of a high density polyethylene (J-REX F6200FD,manufactured by Japan Polyolefins Co., Ltd.) were subjected to meltkneading at 170° C. (residence time of approximately 3 minutes) using atwin-screw extruder (PCM30 apparatus, manufactured by Ikegai Co., Ltd.),and pelletized, yielding 20 kg of columnar compound type pellets of ahigh density polyethylene containing 20 mass % of photocatalytic powder,with dimensions including a diameter of 2 to 3 mm, a length of 3 to 5 mmand a weight of 0.01 to 0.02 g.

(Fiber Formation)

10 kg of the photocatalytic powder containing high density polyethylenecompound produced above and 10 kg of high density polyethylene (J-RexF6200FD, manufactured by Japan Polyolefins Co., Ltd.) were mixedtogether for 10 minutes in a V-type mixer (a RKI-40 apparatus,manufactured by Ikemoto Scientific Technology Co., Ltd.) to form mixedpellets. Subsequently, the thus obtained mixed pellets and polyesterresin pellets (FM-OK, manufactured by Teijin Ltd.) were each input intoa melt extrusion fiber forming apparatus (a POLYMERMAID 5, manufacturedby Chubu Kagakukikai Seisakusyo Co., Ltd.), and at a fiber formationpacking temperature of 300° C., 35 kg of a core/sheath structured fiberwith a thickness of 12 denier formed from a 1:1 mass ratio ofphotocatalytic powder containing high density polyethylene (sheath) andpolyester (core) was produced.

(Evaluation of Photocatalytic Activity)

Next, 10 g of this fiber was placed in a 5 L Tedlar Bag (manufactured byGasTech Corporation) and 60 ppm by volume of hydrogen sulfide wasintroduced into the bag. Subsequently, the bag was irradiated with lightwith an ultraviolet light intensity of 6 μW/cm² at a wavelength of 365nm, using a day white fluorescent lamp (HI-WHITE FL20SS-N/18-B,manufactured by Hitachi GE Lighting Co., Ltd), and after 6 hours ofirradiation, the concentration of hydrogen sulfide was measured with adetector tube (No. 4LL, manufactured by GasTech Corporation). Theconcentration of hydrogen sulfide after 6 hours irradiation was almostundetectable.

(Weather Resistance Test)

The fiber described above was irradiated with 50 mW/cm² light using afade meter (SUN TEST CPS+, manufactured by Atlas Corporation), and after24 hours irradiation the fiber was checked for coloring, but no coloringwas visible.

(Preparation 1 of a Coating Agent)

Next, the photocatalytic slurry described above was diluted by addingpure water until the equivalent powder concentration was 0.5 mass %. Awater dispersed urethane resin (VONDIC 1040NS, manufactured by DainipponInk and Chemicals, Inc.) was then added to the slurry in sufficientquantity to produce a 70% ratio of the urethane resin relative to thepowder within the slurry, thereby yielding a coating agent containing aphotocatalytic powder and a urethane resin. The pH of the coating agentwas 7.1.

Subsequently, a polyester non-woven fabric (6 denier, manufactured byTakayasu Co., Ltd.) was immersed in the above coating agent, and afterremoval from the agent, was squeezed with a roller to remove excessagent and then dried for 2 hours at 80° C. to yield a polyesternon-woven fabric with a photocatalytic powder supported thereon.

(Evaluation of Photocatalytic Activity)

Next, 10 g of this polyester non-woven fabric was placed in a 5 L TedlarBag, and 60 ppm by volume of hydrogen sulfide was introduced into thebag. Subsequently, the bag was irradiated with light with an ultravioletlight intensity of 6 μW/cm² at a wavelength of 365 nm, using a day whitefluorescent lamp, and after 6 hours of irradiation, the concentration ofhydrogen sulfide was measured with a detector tube (No. 4LL,manufactured by GasTech Corporation). The concentration of hydrogensulfide after 6 hours irradiation was almost undetectable.

(Weather Resistance Test)

The polyester non-woven fabric described above was irradiated with 50mW/cm² light using a fade meter (SUN TEST CPS+, manufactured by AtlasCorporation), and after 24 hours irradiation the fabric was checked forcoloring, but no coloring was visible.

(Preparation 2 of a Coating Agent)

A zirconium ammonium carbonate solution (containing 20 mass % as ZrO₂,manufactured by Nippon Light Metal Co., Ltd.) and pure water were addedto the photocatalytic slurry described above, thereby yielding a coatingagent. The coating agent comprised 1.5 mass % of photocatalytic powder,and the ratio of ZrO₂/photocatalytic powder (mass ratio) was 20%. The pHof the coating agent was 8.2.

Next, a transparent noise-barrier insulation wall formed from an acrylicresin board of thickness 15 mm was subjected to hard coat treatment withTOSGUARD 510 manufactured by GE Toshiba Silicones Co., Ltd., yielding atransparent hard coat treated resin board. Measurement of the totallight transmittance using a haze meter TC-III manufactured by TokyoDenshoku Co., Ltd., revealed a result of 86%. The above coating agentwas applied to the transparent resin board using a bar coating method,yielding a transparent noise-barrier wall provided with a photocatalyticfilm on the surface. The DWH value for the board was 37%, the thicknessof the photocatalytic film was 0.3 μm, the total light transmittance ofthe photocatalytic film covered transparent acrylic board was 86%, T2/T1was 0.97, and the pencil hardness was 4H. Furthermore, measurement ofthe contact angles with respect to water revealed values of 20° for CLand 5° for CD. An accelerated exposure test was also conducted using aSUNSHINE XENON LONG LIFE WEATHER METER, manufactured by Suga TestInstruments Co. Ltd., using a BP temperature of 63±3° C., and rainfallof 12/60 minutes. Even after 4000 hours exposure, the contact angle withrespect to water following 24 hours irradiation with light from a daywhite fluorescent lamp producing an ultraviolet light intensity of 6μW/cm² at a wavelength of 365 nm was 8°, and the degree of yellowing was6.

Example 2

With the exception of replacing the 100 g of sodium pyrophosphate (afood additive, manufactured by Taihei Chemical Industrial Co., Ltd.)from the example 1 with 100 g of sodium tripolyphosphate (a foodadditive, manufactured by Taihei Chemical Industrial Co., Ltd.), aphotocatalyst slurry was prepared in the same manner as the example 1.

Measurement of the pH of the thus obtained photocatalytic slurryrevealed a pH value of 7.7. Furthermore, when the zeta potential wasmeasured via an electrophoresis light scattering method using a ELS-8000device, manufactured by Otsuka Electronics Co., Ltd., the isoelectricpoint was 2.0.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. in order to measure the solid fraction concentration of theslurry, and revealed a value of 10 mass %. Furthermore, thetransmittance at 550 nm of a slurry sample of thickness 2 mm was 48%,and the slurry displayed excellent dispersibility. Analysis of thepowder produced above using FT-IR revealed tripolyphosphate absorption.Next, analysis of the dried powder by ICP revealed the existence of 0.8mass % of Na and 1.1 mass % of phosphorus. Measurement of the BETspecific surface area produced a result of 140 m²/g. Furthermore, theDWA value of the powder was 61%. This value is greater than the DWAvalue of the raw material titanium dioxide, indicating that the surfacetreated product offers a higher level of photocatalytic activity.

Example 3

With the exception of replacing the 100 g of sodium pyrophosphate (afood additive, manufactured by Taihei Chemical Industrial Co., Ltd.)from the example 1 with 100 g of sodium tetrapolyphosphate (a foodadditive, manufactured by Taihei Chemical Industrial Co., Ltd.), aphotocatalyst slurry was prepared in the same manner as the example 1.Measurement of the pH of the thus obtained photocatalytic slurryrevealed a pH value of 7.7. Furthermore, when the zeta potential wasmeasured via an electrophoresis light scattering method using a ELS-8000device, manufactured by Otsuka Electronics Co., Ltd., the isoelectricpoint was 1.9.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. in order to measure the solid fraction concentration of theslurry, and revealed a value of 10 mass %. Furthermore, thetransmittance at 550 nm of a slurry sample of thickness 2 mm was 36%,and the slurry displayed excellent dispersibility. Analysis of thepowder produced above using FT-IR revealed tetrapolyphosphateabsorption. Next, analysis of the dried powder by ICP revealed theexistence of 0.8 mass % of Na and 0.9 mass % of phosphorus. Measurementof the BET specific surface area produced a result of 141 m²/g.Furthermore, the DWA value of the powder was 55%. This value is greaterthan the DWA value of the raw material titanium dioxide, indicating thatthe surface treated product offers a higher level of photocatalyticactivity.

Example 4

50 liters (hereafter, “liters” is abbreviated as “L”) of pure water wasmeasured, placed in a vessel and heated with stirring until thetemperature reached a steady 98° C. 3.6 kg of an aqueous solution oftitanium tetrachloride with a Ti concentration of 15 mass %(manufactured by Sumitomo Titanium Corporation) was then added dropwiseto the water over a period of 120 minutes. The white colored suspensionobtained on completion of the dropwise addition was then subjected todechlorination in an electrodialysis device, producing a slurry of pH 4.sample was taken from the thus formed photocatalyst slurry, andmeasurement of the solid fraction concentration by drying to a constantweight revealed a value of 2 mass %. Structural analysis of the driedpowder using an X-ray diffraction apparatus revealed that the productpowder was a brookite titanium dioxide. The brookite content was 89 mass% and the anatase content was 11 mass %. The DWA value for the powderwas 38%.

Next, 100 g of sodium pyrophosphate (a food additive, manufactured byTaihei Chemical Industrial Co., Ltd.) was dissolved in pure water,forming 2 kg of a 5 mass % aqueous solution of sodium pyrophosphate.

In addition, 100 g of calcium chloride (a food additive, manufactured byTokuyama Corporation) was also dissolved in pure water, forming 2 kg ofa 5 mass % aqueous solution of calcium chloride.

50 L of the 2 mass % titanium dioxide slurry obtained above was placedin a reaction vessel and stirred well with cooling. 2 kg of the 5 mass %aqueous solution of sodium pyrophosphate, 2 kg of the 5 mass % aqueoussolution of calcium chloride, and a 10 mass % aqueous solution ofcaustic soda were added to the slurry over a period of one hour, toproduce a mixed slurry with a pH within a range from 8 to 9. During theaddition, the reaction temperature was maintained within a range from 20to 25° C.

The thus produced pyrophosphate containing titanium dioxide slurry wasstored for 1 hour at a temperature of 22 to 28° C. The electricconductivity at this point was 9,500 μS/cm. Next, the slurry wasfiltered and cleaned using a rotary filter press (manufactured byKotobuki Engineering and Manufacturing Co., Ltd.), was then washed wellwith water until the electric conductivity of the filtrate reached 47μS/cm, and subsequently concentrated to produce a photocatalytic slurry.Measurement of the pH of the thus obtained photocatalytic slurryrevealed a value of 7.8. Furthermore, when the zeta potential wasmeasured via an electrophoresis light scattering method using a ELS-8000device, manufactured by Otsuka Electronics Co., Ltd., the isoelectricpoint was 1.8.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. in order to measure the solid fraction concentration of theslurry, and revealed a value of 10 mass %. Furthermore, thetransmittance at 550 nm of a slurry sample of thickness 2 mm was 42%,and the slurry displayed excellent dispersibility. Analysis of thepowder produced above using FT-IR revealed pyrophosphate absorption.Next, analysis of the dried powder by ICP revealed the existence of 0.5mass % of Ca and 1.3 mass % of phosphorus. Measurement of the BETspecific surface area produced a result of 140 m²/g. Furthermore, theDWA value of the powder was 55%. This value is greater than the DWAvalue of the raw material titanium dioxide, indicating that the surfacetreated product offers a higher level of photocatalytic activity.

Example 5

With the exception of replacing the 2 kg of the 5 mass % aqueoussolution of calcium chloride from the example 4, formed by dissolving100 g of calcium chloride (a food additive, manufactured by TokuyamaCorporation) in pure water, with 10 kg of a 5 mass % aqueous solution ofaluminum chloride formed by dissolving 500 g of aluminum chloridehexahydrate (guaranteed reagent, manufactured by Kanto Kagaku Co., Ltd.)in pure water, a photocatalyst slurry was prepared in the same manner asthe example 4. Measurement of the pH of the thus obtained photocatalyticslurry revealed a pH value of 6.9. Furthermore, when the zeta potentialwas measured via an electrophoresis light scattering method using aELS-8000 device, manufactured by Otsuka Electronics Co., Ltd., theisoelectric point was 2.0.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. in order to measure the solid fraction concentration of theslurry, and revealed a value of 10 mass %. Furthermore, thetransmittance at 550 nm of a slurry sample of thickness 2 mm was 36%,and the slurry displayed excellent dispersibility. Analysis of thepowder produced above using FT-IR revealed pyrophosphate absorption.Next, analysis of the dried powder by ICP revealed the existence of 0.3mass % of Al and 0.8 mass % of P. Measurement of the BET specificsurface area produced a result of 140 m²/g. Furthermore, the DWA valueof the powder was 49%. This value is greater than the DWA value of theraw material titaium dioxide, indicating that the surface treatedproduct offers a higher level of photocatalytic activity.

Example 6

Dilute titanium tetrachloride gas produced by diluting 8.3 Nm³/hr ofgaseous titanium tetrachloride with 6 Nm³/hr of nitrogen gas waspreheated to 1,100° C., while a mixed oxidizing gas formed from 4 Nm³/hrof oxygen and 15 Nm³/hr of water vapor was preheated to 1,000° C. Usinga reaction apparatus shown in FIG. 2, these two raw material gases wereintroduced into a quartz glass reaction vessel through a coaxialparallel flow nozzle at flow velocities of 35 m/s and 50 m/srespectively. Following the introduction of cooling air into thereaction vessel to limit the high temperature residence time, at atemperature exceeding 700° C., to 0.2 seconds, the product titaniumdioxide powder was collected using a tetrafluoroethylene bag filter.This powder was then heat treated at 350° C. for one hour. The thusobtained titanium dioxide had a BET specific surface area of 54 m²/g, arutile content of 33 mass %, and an anatase content of 67 mass %. TheDWA value for the powder was 18%.

Next, 50 L of a 2 mass % water based slurry containing 900 g of theabove powder was prepared, and exhibited a pH of 2.3. This slurry wassubjected to demineralization using a negative ion exchange resin, whichaltered the pH to 3.7.

Next, 100 g of sodium pyrophosphate (a food additive, manufactured byTaihei Chemical Industrial Co., Ltd.) was dissolved in pure water,forming 2 kg of a 5 mass % aqueous solution of sodium pyrophosphate.

In addition, 100 g of calcium chloride (a food additive, manufactured byTokuyama Corporation) was also dissolved in pure water, forming 2 kg ofa 5 mass % aqueous solution of calcium chloride.

50 L of the 2 mass % titanium dioxide slurry obtained above was placedin a reaction vessel and stirred well while cooling. 2 kg of the 5 mass% aqueous solution of sodium pyrophosphate, 2 kg of the 5 mass % aqueoussolution of calcium chloride, and a 10 mass % aqueous solution ofcaustic soda were added to the slurry over a period of one hour, toproduce a mixed slurry with a pH within a range from 8 to 9. During theaddition, the reaction temperature was maintained within a range from 20to 25° C.

The thus produced pyrophosphate containing titanium dioxide slurry wasstored for 1 hour at a temperature of 22 to 28° C. The electricconductivity at this point was 9,400 μS/cm. Next, the slurry wasfiltered and cleaned using a rotary filter press (manufactured byKotobuki Engineering and Manufacturing Co., Ltd.), was then washed wellwith water until the electric conductivity of the filtrate reached 40μS/cm, and subsequently concentrated to produce a photocatalytic slurry.

Measurement of the pH of the thus obtained photocatalytic slurry (usinga D-22 device manufactured by Horiba Ltd.) revealed a value of 7.8.Furthermore, when the zeta potential was measured via an electrophoresislight scattering method using a ELS-8000 device, manufactured by OtsukaElectronics Co., Ltd., the isoelectric point was 2.3.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. in order to measure the solid fraction concentration of theslurry, and revealed a value of 10 mass %. Furthermore, thetransmittance at 550 nm of a slurry sample of thickness 2 mm was 8%.Analysis ofthe dried powder by ICP (using an TCPS-100V device,manufactured by Shimadzu Corporation) revealed the existence of 0.2 mass% of Na and 0.3 mass % of phosphorus. Measurement of the BET specificsurface area produced a result of 58 m²/g. Furthermore, the DWA value ofthe powder was 62%. This value is greater than the DWA value of the rawmaterial titanium dioxide, indicating that the surface treated productoffers a higher level of photocatalytic activity.

Comparative Example 1

In a similar manner to the example 1, 50 L of pure water was measured,placed in a vessel and heated with stirring until the temperaturereached a steady 98° C. 3.6 kg of an aqueous solution of titaniumtetrachloride with a Ti concentration of 15 mass % was then addeddropwise to the water over a period of 120 minutes. The white coloredsuspension obtained on completion of the dropwise addition was thenconcentrated under reduced pressure at 40° C., and then subjected todechlorination in an electrodialysis device, producing a slurry of pH 4.When the zeta potential was measured via an electrophoresis lightscattering method using a ELS-8000 device, manufactured by OtsukaElectronics Co., Ltd., the isoelectric point was 4.5. A sample was takenfrom the thus formed photocatalytic slurry, and measurement of the solidfraction concentration by drying to a constant weight revealed a valueof 10 mass %. Structural analysis of the dried powder using an X-raydiffraction apparatus revealed that the product powder was a brookitetitanium dioxide. The brookite content was 89 mass % and the anatasecontent was 11 mass %. Measurement of the BET specific surface areaproduced a result of 140 m²/g. The transmittance at 550 nm of a slurrysample of thickness 2 mm was 44%. Furthermore, the DWA value for thepowder was 11%.

(Preparation of a High Density Polyethylene Master Batch)

A portion of a photocatalytic slurry prepared in the same manner asdescribed above was dried using a media fluidized drying apparatus (aSLURRY DRYER, manufactured by Okawara Manufacturing Co., Ltd.), yielding5 kg of a photocatalytic powder. 20 parts by mass of this photocatalyticpowder, 2 parts by mass of zinc stearate (ZINC STEARATE S, manufacturedby NOF Corporation), and 78 parts by mass of a high density polyethylene(J-Rex F6200FD, manufactured by Japan Polyolefins Co., Ltd.) weresubjected to melt kneading at 170° C. (residence time of approximately 3minutes) using a twin-screw extruder (PCM30 apparatus, manufactured byIkegai Co., Ltd.), and pelletized, yielding 20 kg of columnar compoundtype pellets of a high density polyethylene containing 20 mass % ofphotocatalytic powder, with dimensions including a diameter of 2 to 3mm, a length of 3 to 5 mm and a weight of 0.01 to 0.02 g.

(Fiber Formation)

10 kg of the photocatalytic powder containing high density polyethylenecompound produced above and 10 kg of high density polyethylene (J-REXF6200FD, manufactured by Japan Polyolefins Co., Ltd.) were mixedtogether for 10 minutes in a V-type mixer (a RKI-40 apparatus,manufactured by Ikemoto Scientific Technology Co., Ltd.) to form mixedpellets.

Subsequently, the thus obtained mixed pellets and polyester resinpellets (FM-OK, manufactured by Teijin Ltd.) were each input into a meltextrusion fiber forming apparatus (a POLYMERMAID 5, manufactured byChubu Kagakukikai Seisakusyo Co., Ltd.), and at a fiber formationpacking temperature of 300° C., 35 kg of a core/sheath structured fiberwith a thickness of 12 denier formed from a 1:1 mass ratio ofphotocatalytic powder-containing high density polyethylene (sheath) andpolyester resin (core) was produced.

(Evaluation of Photocatalytic Activity)

Next, 10 g of this resin was placed in a 5 L Tedlar Bag (manufactured byGasTech Corporation) and 60 ppm by volume of hydrogen sulfide wasintroduced into the bag. Subsequently, the bag was irradiated with lightwith an ultraviolet light intensity of 6 μW/cm² at a wavelength of 365nm, using a day white fluorescent lamp (HI-WHITE FL20SS-N/18-B,manufactured by Hitachi GE Lighting Co., Ltd), and after 6 hours ofirradiation, the concentration of hydrogen sulfide was measured with adetector tube (No. 4LL, manufactured by GasTech Corporation). Theconcentration of hydrogen sulfide after 6 hours of irradiation was 12ppm by volume. This result was considerably higher than the resultobserved in the example 1, indicating that the photocatalytic functionwith a day white fluorescent lamp as the light source was considerablyinferior to that of the example 1.

(Weather Resistance Test)

The fiber described above was irradiated with 50 mW/cm² light using afade meter (SUN TEST CPS+, manufactured by Atlas Corporation), and after24 hours irradiation the fiber was checked for coloring, and a strongyellow coloring was observed.

Comparative Example 2

50 L of pure water was measured, placed in a vessel and heated whilestirring until the temperature reached a steady 98° C. 3.6 kg of anaqueous solution of titanium tetrachloride with a Ti concentration of 15mass % was then added dropwise to the water over a period of 120minutes. The pH was 0. A sample was taken from the thus formedphotocatalytic slurry, and measurement of the solid fractionconcentration by drying to a constant weight revealed a value of 2 mass%. Structural analysis of the dried powder using an X-ray diffractionapparatus revealed that the product powder was a brookite titaniumdioxide. The brookite content was 89 mass % and the anatase content was11 mass %. Furthermore, the DWA value for the powder was 11%.

Next, 100 g of sodium pyrophosphate (a food additive, manufactured byTaihei Chemical Industrial Co., Ltd.) was dissolved in pure water,forming 2 kg of a 5 mass % aqueous solution of sodium pyrophosphate.

50 L of the 2 mass % titanium dioxide slurry obtained above was placedin a reaction vessel and stirred well while cooling. 2 kg of the 5 mass% aqueous solution of sodium pyrophosphate, and a 10 mass % aqueoussolution of caustic soda were added to the slurry over a period of onehour, to produce a mixed slurry with a pH within a range from 8 to 9.During the addition, the reaction temperature was maintained within arange from 20 to 25° C.

The thus produced pyrophosphate containing titanium dioxide slurry wasstored for 1 hour at a temperature of 22 to 28° C. The electricconductivity at this point was 22,000 μS/cm. Next, the slurry wasfiltered and cleaned using a rotary filter press (manufactured byKotobuki Engineering and Manufacturing Co., Ltd.), was then washed wellwith water until the electric conductivity of the filtrate reached 50μS/cm, and subsequently concentrated to produce a photocatalytic slurry.

Measurement of the pH of the thus obtained photocatalytic slurry (usinga D-22 device manufactured by Horiba Ltd.) revealed a value of 7.8.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. to produce a powder. Calculation of the solid fractionconcentration of the slurry based on the quantity of powder revealed avalue of 10 mass %. Furthermore, the transmittance at 550 nm of a slurrysample of thickness 2 mm was 16%. Analysis of the powder produced aboveusing FT-IR (an FT-IR1650 apparatus, manufactured by PerkinElmer Inc.)revealed pyrophosphate absorption. Next, analysis of the dried powder byICP (using an ICPS-100V device, manufactured by Shimadzu Corporation)revealed the existence of 0.9 mass % of Na and 1.3 mass % of phosphorus.Measurement of the BET specific surface area produced a result of 140m²/g. Furthermore, the DWA value of the powder was 10%. This value isless than the DWA value of the raw material titanium dioxide.

Comparative Example 3

A test was performed in accordance with the example presented inJapanese Unexamined Patent Application, Laid-open No. Hei 11-278843 A.50 L of pure water was measured, placed in a vessel and heated whilestirring until the temperature reached a steady 98° C. 3.6 kg of anaqueous solution of titanium tetrachloride with a Ti concentration of 15mass % was then added dropwise to the water over a period of 120minutes. The pH was 0. A sample was taken from the thus formedphotocatalytic slurry, and measurement of the solid fractionconcentration by drying to a constant weight revealed a value of 2 mass%. Structural analysis of the dried powder using an X-ray diffractionapparatus revealed that the product powder was a brookite titaniumdioxide. The brookite content was 89 mass % and the anatase content was11 mass %. Furthermore, the DWA value for the powder was 11%.

Next, 100 g of pyrophosphoric acid (reagent grade, manufactured by KantoKagaku Co., Ltd.) was dissolved in pure water, forming 2 kg of a 5 mass% aqueous solution of pyrophosphoric acid.

50 L of the 2 mass % titanium dioxide slurry obtained above was placedin a reaction vessel and stirred well while cooling. 2 kg of the 5 mass% aqueous solution of pyrophosphoric acid was added to the slurry. Inaddition, a 10 mass % aqueous solution of caustic soda was also addedover a period of one hour, and yielded a slurry with a pH of 8.2. Duringthe addition, the reaction temperature was maintained within a rangefrom 20 to 25° C.

The thus produced pyrophosphate containing titanium dioxide slurry wasstored for 1 hour at a temperature of 22 to 28° C. The electricconductivity at this point was 28,000 μS/cm. Next, the slurry wasfiltered and cleaned using a rotary filter press (manufactured byKotobuki Engineering and Manufacturing Co., Ltd.), was then washed wellwith water until the electric conductivity of the filtrate reached 58μS/cm, and was subsequently concentrated to produce a photocatalyticslurry.

Measurement of the pH of the thus obtained photocatalytic slurry (usinga D-22 device manufactured by Horiba Ltd.) revealed a value of 7.3.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. to produce a powder. Calculation of the solid fractionconcentration of the slurry based on the quantity of the powder revealeda value of 10 mass %. Furthermore, the transmittance at 550 nm of aslurry sample of thickness 2 mm was 15%. Analysis of the powder producedabove using FT-IR (an FT-IR1650 apparatus, manufactured by PerkinElmerInc.) revealed pyrophosphate absorption. Next, analysis of the driedpowder by ICP (using an ICPS-100V device, manufactured by ShimadzuCorporation) revealed the existence of 0.9 mass % of Na and 1.3 mass %of phosphorus. Measurement of the BET specific surface area produced aresult of 140 m²/g. Furthermore, the DWA value of the powder was 8%.This value is less than the DWA value of the raw material titaniumdioxide.

Comparative Example 4

A test was performed in accordance with the example presented inJapanese Unexamined Patent Application, Laid-open No. 2001-72419 A.

100 g of 120% titanium trichloride solution (guaranteed reagent,manufactured by Wako Pure Chemical Industries Ltd.) was placed in a 300mL flask and stirred under an atmosphere of nitrogen. With the flaskcooled in an ice bath, 141 g of 25% ammonia water (reagent grade,manufactured by Wako Pure Chemical Industries Ltd.) was added dropwiseover a period of approximately 30 minutes to effect a hydrolysis. Thethus obtained sample was filtered, washed and dried. The product wasthen baked in air at 400° C. for one hour, yielding yellow coloredparticles of titanium oxide. The crystal structure of the titanium oxidewas an anatase structure. The DWA was 18%. This represents a loweractivity than the product of the example 1.

Comparative Example 5

In a similar manner to the example 1, 50 L of pure water was measured,placed in a vessel and heated while stirring until the temperaturereached a steady 98° C. 3.6 kg of an aqueous solution of titaniumtetrachloride with a Ti concentration of 15 mass % (manufactured bySumitomo Titanium Corporation) was then added dropwise to the water overa period of 120 minutes. The white colored suspension obtained oncompletion of the dropwise addition was then subjected to dechlorinationin an electrodialysis device, producing a slurry of pH 4. A sample wastaken from the thus formed photocatalyst slurry, and measurement of thesolid fraction concentration by drying to a constant weight revealed avalue of 2 mass %. Structural analysis of the dried powder using anX-ray diffraction apparatus revealed that the product powder was abrookite titanium dioxide. The brookite content was 89 mass % and theanatase content was 11 mass %. Furthermore, the DWA value for the powderwas 11%.

Next, 100 of sodium pyrophosphate (powder for a food additive,manufactured by Taihei Chemical Industrial Co., Ltd.) was added to theslurry, dispersed, and then dissolved.

The thus produced pyrophosphate containing titanium dioxide slurry wasstored for 1 hour at a temperature of 22 to 28° C. The electricconductivity at this point was 10,000 μS/cm. Next, the slurry wasfiltered and cleaned using a rotary filter press (manufactured byKotobuki Engineering and Manufacturing Co., Ltd.), was then washed wellwith water until the electric conductivity of the filtrate reached 50μS/cm, and subsequently concentrated to produce a photocatalytic slurry.Measurement of the pH of the thus obtained photocatalytic slurryrevealed a value of 7.9.

Next, a sample of the slurry was taken and dried to a constant weight at120° C. to produce a powder. Calculation of the solid fractionconcentration of the slurry based on the quantity of powder revealed avalue of 10 mass %. Furthermore, the absorption coefficient of a slurrysample of thickness 2 mm was 21% at 400 nm, and 6% at 550 nm. Theabsorption coefficient of visible light was lower than that observed inthe example 1.

INDUSTRIAL APPLICABILITY

By complexing a compound inactive as a photocatalyst onto the surface offine particles of titanium dioxide under specific conditions, thepresent invention provides photocatalyst particles and powder capable ofexhibiting good photocatalytic function with a light source of extremelylow intensity, as well as an organic polymer composition, a neutral andhighly transparent slurry and coating agent, and articles with surfaceswhich display photocatalytic properties and hydrophilicity, all of whichutilize the above photocatalyst particles or powder. Accordingly, thepresent invention is industrially extremely useful.

1. A method of producing composite particles of titanium dioxide and acompound inactive as a photocatalyst, comprising the steps of preparinga water based slurry of pH 3 to 5 comprising titanium dioxide, preparinga water based solution comprising a compound inactive as aphotocatalyst, and mixing said slurry with said water based solutiontogether, while controlling pH of the mixture in a range of 4 to 10 toreact and form composite particles of titanium dioxide particles with acompound inactive as a photocatalyst.
 2. A method of producing compositeparticles of titanium dioxide and a compound inactive as a photocatalystaccording to claim 1, wherein a concentration of titanium dioxide insaid water based slurry comprising titanium dioxide is 5 mass % or less.3. A method of producing composite particles of titanium dioxide and acompound inactive as a photocatalyst according to claim 1, wherein aconcentration of titanium dioxide on mixing of said water based slurrycomprising titanium dioxide and said water based solution comprising acompound inactive as a photocatalyst is no more than 5 mass %.
 4. Amethod of producing composite particles of titanium dioxide and acompound inactive as a photocatalyst according to claim 1, wherein areaction temperature between said water based slurry comprising titaniumdioxide and said water based solution comprising a compound inactive asa photocatalyst is no more than 50° C.
 5. A method of producingcomposite particles of titanium dioxide and a compound inactive as aphotocatalyst according to claim 1, wherein said step of preparing saidwater based slurry comprising titanium dioxide includes a process forwet synthesis of titanium dioxide, and does not include a process forproducing titanium dioxide powder from said synthesized slurry.
 6. Amethod of producing composite particles of titanium dioxide and acompound inactive as a photocatalyst according to claim 1, wherein saidtitanium dioxide comprises an anatase crystal form.
 7. A method ofproducing composite particles of titanium dioxide and a compoundinactive as a photocatalyst according to claim 1, wherein said titaniumdioxide comprises a brookite crystal form.
 8. A method of producingcomposite particles of titanium dioxide and a compound inactive as aphotocatalyst according to claim 1, wherein said titanium dioxidecomprises a rutile crystal form and a brookite crystal form.
 9. A methodof producing composite particles of titanium dioxide and a compoundinactive as a photocatalyst according to claim 1, wherein said titaniumdioxide comprises at least two crystal forms of anatase, rutile andbrookite crystal forms.
 10. A method of producing composite particles oftitanium dioxide and a compound inactive as a photocatalyst according toclaim 1, wherein a BET specific surface area of said titanium dioxide iswithin a range from 10 to 300 m²/g.
 11. A method of producing compositeparticles of titanium dioxide and a compound inactive as a photocatalystaccording to claim 1, wherein said compound inactive as a photocatalystis a salt selected from a group consisting of phosphates, condensedphosphates, borates, sulfates, condensed sulfates and carboxylates. 12.A method of producing composite particles of titanium dioxide and acompound inactive as a photocatalyst according to claim 11, wherein saidcondensed phosphate is a salt selected from a group consisting ofpyrophosphates, tripolyphosphates, tetrapolyphosphates, metaphosphatesand ultraphosphates.
 13. A method of producing composite particles oftitanium dioxide and a compound inactive as a photocatalyst according toclaim 1, wherein said compound inactive as a photocatalyst is at leastone compound selected from a group consisting of Si compounds, Alcompounds, P compounds, S compounds and N compounds.
 14. A method ofproducing composite particles of titanium dioxide and a compoundinactive as a photocatalyst according to claim 1, wherein said compoundinactive as a photocatalyst comprises at least one metal selected from agroup consisting of alkali metals, alkaline earth metals, transitionmetals and Al.
 15. A method of producing composite particles of titaniumdioxide and a compound inactive as a photocatalyst according to claim14, wherein said alkali metal is at least one metal selected from agroup consisting of Na and K.
 16. A method of producing compositeparticles of titanium dioxide and a compound inactive as a photocatalystaccording to claim 14, wherein said alkaline earth metal is at least onemetal selected from a group consisting of Mg and Ca.
 17. A method ofproducing composite particles of titanium dioxide and a compoundinactive as a photocatalyst according to claim 14, wherein saidtransition metal is at least one metal selected from a group consistingof Fe and Zn.